5mm is fine if the floor needs any compensation. 3mm is good only if the floor is absolutely perfectly level, otherwise the tile edges can end up being a little high or low. Especially with big tiles, much harder to compensate for an uneven floor.
Our tiler was brilliant, he figured out the right flows from room to room, taking into account slight variances in the room levels.
Nice.
If something's out of line you have to live with it forever and once you've noticed a teeny, tiny error it's impossible to ignore even if it's in a far away corner.
I'm trying to take a step back and wait to see the tiler's opinion because one room is 4m wide so with 5mm grout that's 6 tiles = 390


