I bought a group of tiger barbs a few months ago they were all pretty small at the time.Now two out of the three are huge yet the third is not much bigger than it was when I got it.Will it ever grow or is it the fish equivalent of a runt and will always be small.

Could be the bigger ones are the females as they normally grow bigger than the males. How big is your tank and what other fish are in it.

its a 50g which contains 8 Glowlights,6 neon tetra,4 swordtails,2 cardinals and a large common plec.
The larger tiger barbs are about 4 times larger than the little one.Some of my neons are bigger than this tiger barb.

Could be a small male or a runt like you suspect. It might not live long if thats the case.