So I'm building a retro sort of wheelset with 36-spoke silver Campy hubs and Ambrosio aluminum box rims. Thinking 2.0/1.5/2.0 spokes will be OK for my 175 lbs. but my questions are 1) 3-cross or 4-cross lacing? and 2) 2/1.8/2 spokes for the rear drive side?

1. Three or four cross is really a function of hub flange diameter. Sometimes the spokes cross at the spoke holes on the flanges and make it hard to lace. Practically, I only use 4 cross for 40 spoke wheels.

2. If I were building the wheels, especially a "retro sort", I'd use 2/1.8/2 for everything. I don't see that you're gaining anything by using the skinnier spokes.

I use straight 1.8 on my front wheels and straight 2.0 on the rear (sometime DB on the non-drive side) and I weigh 205 or so.

3 cross is fine.

3 or 4 cross is of marginal significance on small flange hubs and only slightly less marginal on LF hubs, but I always built my LF 36h wheels 4x. 4x will not cause any issues on any hub of 36h or more regardless of flange size, so that's your choice.

Your choice of spokes is virtually identical to what I've used for years, going with the heavier 2.0/1.8 butted on the right rear flange to mitigate the effects of dish somewhat.

You have it dialed in perfectly, so toss a coin about the number of crosses and go to it.

the 4th cross is spoke A , crossing spoke , B , then going to the rim opposite to the side of the 1st crossing spoke..

Happened to build a Campag Hi Low Hub , and laced it to exaggerate the spoke length difference.

High Right side 3X, the low flange side 4 cross on the left.. its been fine for 25+ years.. plain 1.8mm (15 gage) un butted.

FWIW, racers when they are doing the cobbles in the race to Roubaix , they pull out the 3 cross box section Tubular rim wheels. unless their sponsor pays the team to use the carbon stuff ..

So, 3 cross is a little shorter spoke than 4, the distance to the right flange of a dished wheel uses a shorter spoke than
to the left flange.. mixed crosses are OK
In fact people have used the fact that 36 is evenly dividable by both 3 and 4,

to lace wheels in a group of 3 , a radial spoke and 2 tangential spokes [2x?] in a group rather than a group of 4..

Yea toss a coin , several , you can build 3 x drive and 2 cross the left side and so forth ...

My first pair of 700c's I built using 4x cross drive side, 3x non drive side. Still true and ridable.

All my BMX wheels I built using 4x with all the spokes radiating to the outside of the hub. We used to call it Race-Lace and it built a nice set of wheels based on 36 spokes, but man calculating the spoke lengths was a pain and more than once I had to recut them after lacing with the Phil machine which I dearly miss being able to use)

3X and I would use 18-16 double butted spokes all around.