In case this has not already been brought to your attention, the experience of the group is that splitting the dose may not be as effective as giving a full dose all at one time. Going from 2 mg all at once to 3 mg but as a split dose (1.5 mg twice a day) might not be more effective than the 2 mg. In fact, it might be less effective. Since you posted on April 23, by now you should have seen some improvement.
As far as I know, the only reason to change to a split dose is if you have seen side effects such as loss of appetite or symptoms of "veil" from a full dose, or you want to add extra pergolide when you are already on a very high daily dose to achieve better control. On this list, 2 or 3 mg of pergolide is not considered a high dose for a horse, or even a pony, and is usually well tolerated (see the pergolide database). We have a 12 H 450 lb pony on 4.5 mg pergolide who was titrated up in September from 3 mg by adding 0.5 mg pergolide every 3 days with no side effects or veil. He has stayed at that dose without side effects.
If you want to switch to a full 3 mg dose once a day, you might go to 2 mg plus 1 mg for 3 days, then increase one dose and decrease the other in increments of either 0.25 mg every 3 days (or 0.5 mg every 3 days if he will tolerate that). Alternately, go back to the 2 mg once a day, and increase with 0.25mg which is the list protocol (or 0.5mg) every 3 days.