Why was 3...c5 for black was a bad move? I played a player ranked about 450 points higher, and I was out-played. I managed to give a fight but was trailing all along.

The player used the English opening and on third move I (black) played c5. My aim was to control centre, and dissuade white from playing d4.

So ,I thought it was a good move, but upon doing analysis with computer it calls it an inaccuracy. Can anyone share why it is an inaccuracy, i.e. why is it a bad move?

[FEN ""]
1. c4 b6 2. Nc3 Bb7 3. e3 c5 4. Qc2 e6 5. Nf3 h6 6. b3 f5 7. Bb2 Nf6 8. O-O-O d6
9. d4 Nbd7 10. Nh4 g5 11. Ng6 Rg8 12. Nxf8 Rxf8 13. f3 g4 14. Nb5 Nb8 15. dxc5
d5 16. Nd6+ Ke7 17. Nxb7 Qc7 18. c6 Nxc6 19. Ba3+ Kf7 20. Bxf8 Kxf8 21. cxd5
Nxd5 22. Bb5 Nxe3 23. Qxc6 Qxc6+ 24. Bxc6 Nxd1 25. Rxd1 Rc8 26. Rd8+ Rxd8 27.
Nxd8 Ke7 28. Nb7 h5 29. Kd2 Kf6 30. f4 e5 31. Ke3 e4 32. Be8 h4 33. g3 hxg3 34.
hxg3 Ke7 35. Bg6 Kf6 36. Bh7 Ke6 37. Bxf5+ Kxf5 38. Nd6+ Ke6 39. Nc8 Kd5 40.
Nxa7 Kc5 41. a3 b5 {&lt;br /&gt;&lt;br /&gt;&lt;em&gt;Game may have continued...&lt;/em&gt;} (41... b5
42. Kxe4 Kd6 43. Nxb5+ Ke7 44. f5 Kd8 45. a4 {+25.6}) 1-0
IMO, 3...c5 by itself, is not really a problem, after the strongest continuation  4.d4 stockfish gives 0.5, and after 4th move in the game stockfish gives equality. And I think your reasoning is fine, it is generally good to trade a c-pawn for d-pawn.

But it looks like it is a part of bigger problem: You made too many pawn moves 5...h6 And 6...f5, and later 8..d6, while not developing your pieces and getting your king to safety. Also by making these pawn moves you created weaknesses, for example your opponent was able to exploit weakness on g6.

Make sure that you focus on developing your pieces and getting your king to safety, and avoid making unnecessary pawn moves.