Do years in service in the Reserves count towards years in service on active duty in terms of pay?

You are getting ripped off. My LES shows that I have 20 years of service. Of those 20, only 4 were active duty. I am currently on active duty orders and am getting paid as a E-8 with over 20 years. So you should be getting credit for your reserve time.

Yes, your Reserve time counts as Time In Service. It should be counted from the day you sign the contract. OTOH, switching to Active Duty will generally reset your Date of Rank. Yes, you need to fix it. All it should take is a copy of your original contract or your dd214 from the Reserves. Or an NGB22 if you were National Guard.
If you were Guard, be aware that the NGB system and HRC system don't talk to each other, so you need to get copies of everything from the Guard side, including quals and retirement points and make sure they are listed on your current record.

Yes your TIS (Time in Service) which is what pay goes off should be 3 years. As long as you were on contract that counts doesn't matter if it is Active, Reserve, IRR you still get that for TIS now your retirement pay will be totally different but TIS would be 3.
I have 8 years active 2 years in IRR and 7 reserve my LES reflects 17 years which is proper. This is something you defiantly need to go fix. Just like your PEBD should be the same date that you shipped to BCT or first day serving which mine shows AUG 22 2001.
If I was you I would defiantly go get your records fixed you should receive back pay also.

The records should reflect every service point or day. The records should reflect your service.

They should have a record of your service. I think you may be able to go to hrc.army. mil I'm be able to go into the Army Reserve self service portion and be able to pull up your points for those three years. I'm sure someone here will correct me if I'm wrong but those records are not lost.

1506 is the form to fix your TIS for pay. You need to have all your supporting documents. Your Career Counselor should be able to help.

Your guard or reserve time is counted by 4 points for drill. 1 point for each day of annual tour now compare that to active duty which is 365 points a year

Yes, it counts towards longevity (TIS) pay. a VERY small fraction counts towards retirement (take the number of points you earned, divide by 360 [yes, 360 not 364] and that's your equivalent years AFS for retirement purposes).

I would double check that you are/are not being paid the correct longevity pay - you may be seeing AFS listed on your LES. Since you said "almost 3 years", your base pay should be "over 2", or $2248.50/mo. If it is, you're good. If it's $2139, see finance with verification of your reserve time.

So every drilling year is counted by a point system. They convert the point system down to days or months. Active duty.tracks years of service by days 365 minus your drilling days approximately 40 days so that's 325 active duty days to.make for every year you were guard or reserve so you could lose pay scale and rank.depending on base and branch.of service.

It's an easy fix. It should have been done at the MEPS.
Take your reserves contract, all your
orders for AT, any DD214s, retirement points worksheet, and your latest enlistment contract to your Career Counselor who will do your 1506 and correct it for you.