There's some odd advice here.

3 hours is by no means comfortable, but I certainly wouldn't say a blanket 'not enough time'.

BA's 35 minute 'conformance' point is just before you enter security, not after it, and there is generally no queue to get to it. Any hold up in security is fine, and not counted against you (though you might have to rush to the gate).

Similarly, the check-in desks close at 45 minutes, so as long as you are at a desk by 1054, you will be fine.

The 40 minute journey from T3 to T5 is a worst-case scenario. Walk a bit faster, and arrive just in time to catch a train, and it will be closer to 20 minutes. So an average of 30?

So your 1h20 window to disembark, clear immigration and collect baggage has just gone up to 1h50. On a good day, that little lot might only take 40 minutes, giving you a good hour's leeway. With that amount of leeway, you can afford a couple of things to go wrong.

Like I said, I would certainly not call it comfortable, and I would make sure I had a backup plan, but 3 hours is by no means out of the question.

If you can persuade AC to tag your bags through to the BA flight (I believe this is more likely than persuading BA to do you the same favour on the return), then so much the better. You can connect by airside bus and avoid immigration.

Similarly, if you avoid checking a bag, and can print your boarding pass online, you can do the airside transfer:

http://www.heathrowairport.com/heath...-international

I would certainly consider that, particularly if buying a new flight/ claiming on insurance is not going to work for you in the event that you miss the GVA flight.

A final option - and not one that I recommend, although I have successfully done it - would be to abandon your bag in T3 if short of time, and collect it on the return. You run the risk of the bag disappearing forever, of course...