Along the lines of "how small of a ship could I do a lvl3 in" ... here's this.
The obvious way to play this might be in an Assault Frig. Honestly, I've struggled with them over the years. And I'd trained Interdictors awhile back but never got to use one in anger.
I came up with this fit out of what I had sitting in the hanger, and I am not a fitting guru. It could probably be min/max'd a little bit more, but it works as is it sits. So with a little bit of fiddling, here's this:
[Heretic, Silly Lvl3 Boat]
Ballistic Control System II
200mm Reinforced Rolled Tungsten Plates I
Small Armor Repairer II
Experimental 1mn Afterburner I
Cap Recharger II
Phased Weapon Navigation Array Generation Extron
7x Light Missile Launcher II
[empty high slot]
Small Bay Loading Accelerator I
Small Warhead Calefaction Catalyst I
I don't claim to be able to do any lvl3 in this ship. I don't claim it to be the most fastest isk or lp/hr. That's not the point. But it is pretty fun, and the Heretic is a pretty good looking little hull.
So if you're bored ... go play. It's your sandbox, have fun.