I have a set of Bowens Gemini 500's (2) but want a 3rd fill in light, will any flash head work with these? ie flash along with them? don't need it to be a 500 and can be a cheaper light, I take it if they flash on activation of the first one then all should sync automatically? Thanks in advance..
In general you can't sync TTL flash units like the 580s with studio flash, partly because of their pre-flash that'll trigger the studio lights but also their infra-red communication between each other and the camera can trigger the studio flash also (as most are IR as well as visible light sensitive).
If you do want to use speedlights with studio flash, you need to run them in full-manual mode and preferably trigger everything with radio triggers to avoid these various issues.
Also bear in mind however that speedlights can be different colour temperature than the light from studio flash (In my experience they markedly cooler).
Try one of these: https://www.amazon.co.uk/Yongnuo-YN-560-Speedlite-Olympus-Cameras/dp/B00PIIRO1C/ref=sr_1_1?ie=UTF8&qid=1481378231&sr=8-1&keywords=yongnuo+560 I have one,it's fully manual and has it's own built in slave cell to trigger the flash.