Yes the AR can be used in SH3, only it is a bit more difficult because the scope marks are not well placed.
As for calculating it, real life is keeping me too busy lately, but I hope to have soon ready a wiz wheel specifically created for calculating the AOB based on the aspect ratio method. I'm currently using a crude version of it, and I can calculate AOB with just one observation in roughly 5-10 seconds

It is the perfect complement to the MK3 sub attack course finder or to the german KM wiz-wheel and it will come in two versions also, KM and Silent Service...when I have time to finish it!
Soon