That doesn't make sense.... if you have three understrength units that are enough to merge to make one then they should merge to make one. Hmm. Try different conbinations of merging, sometimes it works that way. If not, get a new computer :)

Keep your old troops. I once asked this question too, and I eventually learnt that you can either a) use the troops and continue an offensive, or b) halt them all at the frontier and use them as limes garrison troops while you build up new armies of post-marians, upon which you trot out the principes and use them as flank guards (or the centre) in battles, depending on your tactical plans, and take out the triarii as anti-cav troops.