How many birdhouses can you put in each tree?

[womble]

I've already got one birdhouse for something like bluetits in a fairly big apple tree.
Can I fit another two in there, they would be about 2 and a half to three metres apart.

[Guttata]

One of my friends has put 6 in a tree, a mix of types and he says they are always taken up. Though it is a beech tree so I am not sure how that would compare to an apple. Try it and see.

[WrexTheDragon]

I have 3 up a Silver Birch about 1ft apart and it does not stop them nesting.

One has a family of three pairs of bluetits sharing one box.

[womble]

Thankyou! Seems easy then.