Look into Sensory Integration Disorders and possibly PDD NOS or Asperger's Disorder.
Stop the singulair! Look it up on the web - there are all new reports that it causes all kinds of problems. My son was on it years ago - and he had a grand mal seizure - and the doctors laughed when I said I thought it caused it. I stopped it - and he never had another one!! Amazing, huh??
Your son seems awfully young to be expected to behave well at church. I can't think of any 3 year old that does.
He likes things that spin-- I think thats ok, too-- it when thats ALL HE likes and when he does not want to play with ANYTHING else that you have a problem.
Singulair-- most definitely allergy/ asthma medications can cause hyperactivity. Not sure about its dangers as previous post mentioned -- but it absolutely can cause behavior changes in children. Without question.
Get him off the Singulair. You might want to try Zyrtec. My son had bad allergies as a small child - he was later found out to be gluten intolerant. Artificial colorings have now een definitely proven to trigger off alot of these symptoms. Whatever he is doing - cheer up - my kid was so ADHD off the charts he was kicked out of public school (and he was a straight A student). Now, before you want to start hiring lawyers to make the state pony up for the schools that are not on your state list (the ones they pile up all the smart, crazy AHD'ers - it will cost you alot of heartache and a small mint) try to figure out if he has a food allergy too. There are elimination diets to go by. You want to keep off the aphentamines at this age - it stunts there growth.
If you truly believe it is ADHD - then get prepared for elementary school. Get letters, inform the school right away, ask for an IEP....
But try to look at his diet first - it took us 8 years and a very good Celiac doctor to figure it out. At your son's age he was just borderline on the gluten - but by 8 he was way over the line and his behavior improved greatly when the offending allergen was removed.
Kids that age are very active, very curious, and do not have high attention spans. At least most of them don't. Whether they are on meds that cause this or are naturally that way, they can not be expected to be little adults and behave perfectly at all times.
I'm not saying you expect your child to act like an adult, but the way you said he runs around the house at "inappropriate times" and how he tends to not pay attention while at church, learning something new, or doing a workbook...that suggests to me that he is bored or maybe these things are not being presented to him in a kid-friendly manner.