Look up "minor losses" in the pipe-flow section of your fluids book. dp = K * dynamic head.
K = minor loss coefficient; depends on open area ratio for your grate, at least.
dynamic head = (1/2) rho * V^2
rho = density of fluid (air)
V = average velocity -- use velocity approaching the grate unless the data you find for K specifies otherwise.
Using V = Q/A lets you build a resistance curve for fan selection. Q is volume flow rate and A is the grate overall area. Watch the units, it's easy to get tangled.
Are the three grates in parallel or series?
If series, add the pressure drops. If parallel, assume evenly distributed flow between them, and calc the pressure drop based on 1/3 of the total flow rate.