Wheat DME is a blend of base malt (probably a two row brewers malt) and wheat malt; the ratio varies based on the brand, let's assume it's 50% wheat & 50% base malt.
Light DME is probably a "two row brewers malt".
Let's assume that DME is 44 gravity points per pound.
Let's assume that your BIAB efficiency is 75%. Each of the malts will yield 27 gravity points per pound.
3# of wheat DME is 66 gravity points of wheat malt (3 * 44 * 50%) and 66 gravity points of light DME.
You need 66 gravity points of wheat malt and 110 gravity points (66 + 44) of "brewers malt".
66 / 27 ==> roughly 2.5 lb of wheat malt
110 / 27 ==> roughly 4.0 lb of base malt.