Is there any trick to use the lounges in Terminal 3 in LHR? I arrive with my partner with AA in T3 and will continue with BA from the T5.

Because I like to try the Cathay Pacific First Class Lounge as OW Emerald.

Assuming you are in transit on a single ticket, you don't get to use the lounges at T3 without a boarding pass for T3. You'll get to use the BA First Lounge at T5, that's it (besides the BA Galleries ones there of course).

There is no airside transit at LHR. You need a boarding pass for that terminal to access the departure area for that terminal and the only way to transfer terminals is via the arrivals stream.

You'll arrive in T3 on AA, and will go into the arrivals passage where you can either go to immigration or flight connections. At flight connections, you will get on a bus to T5 (and will need a T5 boarding pass to get on the bus), which will then drop you off at a corridor leading to security.

The only T3 lounge you would be able to use is the AA Arrivals lounge, assuming it is open when you arrive and you have enough time. You'll have to pass through immigration and enter the UK.
Page 5 - https://www.heathrow.com/file_source...row_T3_Map.pdf
You exit immigration through the world duty free shop labeled 3 on the map. The Arrivals lounge is above the cafes labeled 1 and 2.
After some time in the lounge, you then go to the train station and use the Heathrow Express train to transfer to T5 (Heathrow Express is free between the airport stations).

Arrivals lounges aren't part of the oneworld agreement, so have their own rules for entry. However, OW Emerald having arrived in LHR on an AA flight will get you in.
The only other lounge options you would have are the BA lounges in T5.

Yes there is, and in fact the route you described to the OP going on the bus from T3 to T5 is a completely airside transfer. I think what you meant (and what you refer to in your post) is that there is segregation between arriving and departing passengers and you can't freely pass between the airside arriving and airside departures streams.

Not true for T3 actually. It is true for T5 though, you can't go in to the airside departures area in T5 without a bp for a T5 departing flight. There is a way for the OP to go in to the airside departures area in T3 and then exit to go to T5, but as the T3 lounges themselves may be likely to refuse access to someone not departing T3 I won't go in to details - although it's been mentioned before many times on the UK board and BAEC board.

You do not need to present any bp when getting on the airside buses - the first place the OP would need to show a bp is at flight connections in T5. In fact you can use this to your advantage so if arriving at T3 and it would be better for you to get to T5 (perhaps you are staying in the sofitel attached to T5 for example) you can can actually follow the signs for flight connections T2/3/4, get the airside bus to T5, and go through immigration at T5.

Anyway, I do agree with what you and others have advised overall, the OP should just follow the flight connections route to T5 and use the GF lounge there.

Just used my T5 boarding pass at Cathey Pacific T3 lounge
no question asked ....
So it's possible

How do you get airside at T3 with a T5 boarding pass?

I working at LHR....
but You can also do personal shopping assistant trick...

It probably won't work using the personal shopper sevice.

https://boutique.heathrow.com/en/page/terminal-transfer

Still unclear as to whether I can head to T3 lounges arriving T3 but departing T5 - I have a 7 hour layover.

Last month I arrived T5, transited to T3 for my departure flight. However, I had invited the chap seated beside me to join me in the lounge at T3 on the understanding he was also departing T3. As we stood in security before entering departure area in T3 he confirmed in fact he was departing T5. He passed through before security showing his BP, went through security and joined me in the BA T3 lounge. No questions asked. Do not know what became of him as 2 hours before his T5 departure I recommended he start on his return journey to T5...when I left him he had an hour to get there...I doubt he made. He seemed to have been enamoured by the offerings from the buffet.

I will clear security in T3 in order to enter the departure area ostensibly to shop and or visit a lounge and then return to the bus for transfer to T5. It appears that the lack of a T3 boarding pass is not the issue rather that I have enough time to return to catch my flight 7 hours later. I will report back and update.

I fly through T3 regularly, and I have never once noticed the possibility of going from the T3 Departures level to the Arrivals level and/or Flight Connections.

Which isn't to say it doesn't exist, but you will need help finding it...

You have been able to to do it for a while. Slightly changed now with the new FCC which opened in May 2018, but it is all details here https://www.flyertalk.com/forum/29998308-post383.html you can freely pass from the T3 airside departures area either to immigration & landside or to the airside connection buses to T2/4/5.

I have done either arrived at T3 and T5 with a flight leaving from T5, and gone to the T3 departure terminal many times over the last couple of years. Sometimes they question why, and I just tell them I am going to the lounges. Cathay Pacific is my favorite Lounge at Heathrow. They have never questioned my access regardless of the terminal where my flight leaves.

Are there any recent experiences on visiting T3 while connecting through Arrival in T5 Departing in T5 flight connection. Have a 6 hrs window in Nov.

See this thread in the BA forum which is much more active and up to date:
https://www.flyertalk.com/forum/british-airways-executive-club/1920657-switching-terminals-lhr-lounge-use-8.html