First, welcome to the forum.
Second, you are not under a disability under ORC (2923.13), you are under a federal disability as explained by mreising.
Third, you CAN apply for a relief from disability in the county in which you were convicted (2923.14). IF granted, that would fix your federal disability as BobK mentioned.
THEREIN LIES THE PROBLEM.
It's completely up to the judge to decide. Got an anti-gun judge? Ain't gettin' relief.
Best you can do is get a GOOD lawyer (preferably a GREAT lawyer) to write up and file your application.
(insert shameless plug for Derek DeBrosse at http://www.ohiogunlawyer.com
As lrp3rd said, it's probably $200 or more just to file the application (no official 'application' form. It's filed like any other lawsuit.) A lawyer will add an extra '0' to the end of that price.
What's the difference between a GOOD lawyer and a GREAT lawyer?
A GOOD lawyer knows the law.
A GREAT lawyer knows the judge.
If you haven't done any volunteer work in the community in which you are seeking relief, it's not too late to start. Might want to join a church, synagogue, mosque, etc.
In which county were you convicted? You made it seem like you were charged with both crimes at the same time. You said you served your full 3 years. Was that all prison time or was there some parole/probation time?
How old are you? Do you have a juvenile record that has NOT been expunged?
Rather than answering all these questions, do you know the case numbers from your charges?