Based on your wording "according to theory, 3...Nc6 is black's best reply" I'm inferring that you don't want unsound suggestions that may offer you practical chances over the board, but may not be correct with best play.
Probably some of your opponents will go 3.Nc3 because they don't know the Petroff, even though they are not Four Knights players. So I would recommend the Rubinstein variation 1.e4 e5 2.Nf3 Nf6 3.Nc3 Nc6 4.Bb5 Nd4, which is a tactical line where White can get into trouble if he doesn't know it. In the main lines Black does at least as well as in the Petroff.
The Scotch Four Knights with 4.d4 is another matter but the main line should be fine for Black and once again, many players as White won't know it very well.
You may not like the Guioco Pianissimo, which comes after 4.Bc4 Bc5 5.d3 d6 because you'll need toothpicks to keep your eyelids open. So you can instead consider 4...Nxe4!? 5.Nxe4 d5. (But if so, then had better study 5.O-O!? to make sure you don't get in trouble in that line.)
If instead you are like me, then you'd just intensively study something crazy like 3...d5?! and learn all the ways that White can go wrong in that line. (Maybe 4.Nxe5 d4 5.Ne2 Nxe4 with a good game for Black.) Then you'd be playing an unsound line where White can get an advantage, but you'd win a lot of games against inaccurate play by your opponents who have no idea what to do. However this approach is not for the faint of heart, so don't try it if you're afraid of losing.
A few months back, I chose the Petroff as my defense to 1.e4 2.Nf3.
After 1.e4 e5 2.Nf3 Nf6, my opponents have been playing 3.Nc3, and according to theory, 3...Nc6 is Black's best reply, which transposes the Petroff into the Four Knights.
I don't have any sharp lines worked out in the Four Knights, anything that would present problems for White to solve in a 15 10 game.
Can Black give White a headache in the Four Knights? Are there any sharp replies?