There are genetic mutations that can cause a MJ plant to have 3 leaves. These are somewhat rare but happens occasionally. Of these, the most common type is called a whorled phyllotaxy. The plant should be able to grow out fine, but potency might be affected.
the first set of laflets might have 3 leafs maybe even the second set but as it gets bigger, u will get the 5 7 and 9 maybe even 11 leaflets the bigger it gets seeing as u r a beginner i was thinking u had never seen the plant grow before!!!
^Actually that's how I read it too, but now looking back at the original post I'm not so sure. To the OP, do you mean a MJ plant that has 3 rather than 2 fan leaves at each node, or do you mean that the fans on your plant have only 3 blades each?