Here's some series contributions/ impacts in Asia. If I'm misjudging series feel free to correct. Contribution is the role they played not dependent on result, and impact is the difference their contribution made to results. It's zero in a lost series and zero unless they perform in a won game.
McGrath:
Lanka: one series, lost one. Poor contribution. No impact
India. Three series. Lost two, won one. Contributed significantly in two. Major impact in the win- balanced series.
Pakistan: two series. Won one and lost one. Reasonable contribution and fairly low impact in the win.
Plus UAE: one series, one win. Major contribution, low impact on the series result (mismatch).
So he's lost one more series than he's won, contributed in one more series than not, and has been fairly limited in swinging series resuls.
Marshall:
Marshall has played 5 in total. Against India he won and contributed well without much impact on result in 1 (walkover), lost and contributed poorly in one.
Played three against Pakistan. Won one drew two. Contributed ok or better in all, had a solid impact on series result in all.
Won more than he lost, contributed in almost all and impacted on most.
Steyn:
He contributed significantly and had a major impact in series results in Lanka, Pakistan and India *2. Good contribution and impact in UAE in one series. Major contribution but little impact in Bangladesh. Didn't impact or contribute much in UAE and Lanka. Had a couple of series that he barely got a chance to bowl in.
Steyn averages 4.5 wickets a game to Marshall and McGraths 3.7, which is huge. 7.4 wickets per game in wins, with Marshall at 6 and McGrath at 4.9. Marshall and Mcgrath had more competition, but not by this margin. There isn't much doubt to me he was the best in Asia- even if you ignore the stats the miracle spells should do it. But in terms of impact on series results, there isn't that much difference between him and Marshall.
If the idea of impact on series seems overstated because it's a team game, flat pitches determined a lot of results etc., bear in mind that almost all of these series were won by one game or drawn. Performances in a single game(s) either did swing the series, or could have if they had contributed in a game that they failed in. All 3 played a very similar amount of games with a similar amount of wins and although number of draws differ, that is more of a batting/era factor.