Instead of +1 to three different ability scores at 4th, 7th, and 10th level, why not just say at every level you can increase an ability score by +1 and can not increase the same ability until you advance to 4th, 7th, or 10th level? It works out basically the same but dishes out the advancement more often.
So at 2nd level you gain a +1 to Strength, you can't take another bonus to strength until you are 4th level.
So at 2nd level you gain a +1 to Strength, you can't take another bonus to strength until you are 4th level.