Philidor Defense:
1. e4 e5 2. Nf3 d6 3. Nc3 c6 4. d4 exd4 5. Nxd4 turning the game into an inferior Sicilian where Black has no pawn majority, and has already committed to e5.
3.Nc3 is inferior as is 3...c6.
1.e4 e5 2.Nf3 d6 best is 3.d4, and after 3...exd4 4.Nxd4, best here is not c6 by any stretch of the imagination. Black should go into the Nf6/Be7 lines, or else fianchetto the Bishop (Lasker's Variation).
After the inferior 3.Nc3, Black should play 3...Nf6! where after 4.d4, Black has the Hanham Variation after 4...Nbd7, which is fine for Black!
After 3.d4, 3...Nf6 should be answered by 4.dxe5! and after 4...Nxe4 (all other moves are worse), 5.Qd5 gives White a significant advantage.
This is why most Philidor players have now been reaching it via the Pirc move order. After 1.e4 d6 2.d4 Nf6 3.Nc3, if they are willing to deal witht he endgame variation, then 3...e5 is played. White has no advantage after 4.dxe5 dxe5 5.Qxd8+ Kxd8, but some players just find the line outright annoying. The other option is 3...Nd7, where he will answer 4.Nf3 with 4...Nf6, reaching the Hanham Variation, which is fine for Black. However, Black must be willing to deal with a number of "Anti-Hanham lines" if he chooses to play 3...Nd7, like 4.f4 and 4.g4.
Philidor Defense:
1. e4 e5 2. Nf3 d6 3. Nc3 c6 4. d4 exd4 5. Nxd4 turning the game into an inferior Sicilian where Black has no pawn majority, and has already committed to e5.