The A1b single bookshelf works as a vertical or horizontal center. The planar tweeter can be rotated for horizontal use.
The A3c would work with either the A1b center or A2c center. The A3c is a 2.5way tower, top portion is just an A1b and the bottom woofer is for extra bass. The A1b would work fine with the A5 tower a few users are using it that way.
Arx A5 fronts and A1b center would be around $1000-1100 depending on your shipping options (which you can add everything to the cart and get real time quote) and that would leave enough for a SVS PB12 NSD, HSU VTF3Mk or PSA XV30 and come in a few hundred under your budget.