You remain Lifetime PLT. There is no lifetime EXP.
You get some new luggage tags with the 3MM in the circle. And a new EXP card.
You get 4 additional threshold eVIPs (just like at 2MM and 4MM and 5MM and all subsequent MM thresholds) that are the same as your EXP ones. They may take up to two weeks to post as this is a different cycle than the EXP qual/requal processing.
You are better than a 2MMer in your mind! Tho not in any other priority (ala upgrade list) standpoint!
You will also need to change your special EXP/2MM handshake and co-member greeting. This comes in the Congrats Package.
Thinks thats about it. Now REMEMBER when you get your new luggage tags, when you put the item in the overhead compartment, make certain that the 3MM luggage tag is clearly visible to all others as they walk thru the cabin. Even better, work it so that the tag hangs out of the overhead a little bit and the FA has to move it so that the bin can be shut!