I guess you mean "when to apply a first mulch on the pasture".
If using polythene mulch, of those three months, early March is best, to allow time for pasture to die, and if you spread 1-2 inches manure/compost before covering, that makes a nice surface by late May for planting.
If using 4-6 inches manure/compost on pasture, weeds etc, you can spread it the day before planting if needs must, though I prefer a week or so for the organic matter to settle.
Many options and not just a question of timing.