To be honest I do not know the specific reason why it happens, or if it is normal for some strains, but Im sure someone will answer that part. I can answer that...it doesnt mean the plant is immature or wont produce bud. But it will probably always have 3 leafs. A friend of mine is growing a few clones that are 3 leafed. They went from 3 for awhile, to one set of 5, back to 3. They are about to go into flowering pretty soon so well see if that changes anything. But for about a month old, they are pretty big, so it doesnt seem to affect growth. They are AK-47 and... cant remember the other one.

Ive never liked it, but Ive always cloned leafs that have 5 or 7 so thats what I have always worked with. Once in awhile a 3 shows up here and there.

PS. If you can get your Urkle to turn out good...bah...that is one of the best strains I have ever used.