Heaspace - Primary Fermentation of 3g batches in the 'Ale Pale'

I only have two 5gal carboys and would like to do 2 mini-brews in them. I could purchase another carboy for transferring to secondary, but I just can't justify the $15-20 for another small carboy that won't be used too often.

Is primary in one of the (I think 7.5 gal?) standard 'Ale Pale' going to have too much headspace to primary a 3gal batch of brew?

I'd like to just start experimenting wildly in small batches in case they suck, but am concerned about wasting money on a 'useless' carboy.

primary should be no problem at all.