Can I have a touch up after 3.5 years of having PRK to reduce/eliminate astigmatism?

I recommend a visit to the doctor to check your visual acuity. If it is just a minor prescription left over, a touch up may not be worthwhile due to recovery time and risk of haze. However, if the vision is poor, your doctor would analyze your corneal maps and remaining thickness to make sure it is safe to perform an enhancement. Gaining 20 lbs shouldn't affect refractive error, unless it is related to a health condition like diabetes. 

A touch up could be a possibility post-PRK.  However, as was stated at your 6 month follow-up, the prescription and gain from a surgery has to be sufficient enough to be able to perform a second surgery.  Your surgeon will want your eyes to have hit a stable point to where that distance vision is not changing.  Other factors such as corneal thickness and curvature will also be a variable as to whether another procedure can be done.