first, those aren't spdif, they are just RCA jacks. You can get a 3.5mm to RCA jack for a dollar at any dollar store. If it's red and white, its an analogue RCA jack signal, split between left and right channels. If it's yellow and on an audio card, it's SPDIF and can carry a surround signal over a single cable.
I called them the wrong cable. Actually they are optical cables and the only RCA cables I find are the RED and WHITE ones and some weird one that would never be able to go into the port on the sound card I have.
The reason I want OPTICAL to 3.5mm is because thats what the Astro A50 uses. Thanks for trying I guess.