I'd be careful with Benzoyl Peroxide, generally I'm against such harsh topical medicaments but some people report good results.
My suggestions is - shower twice a day, one in the morning and one time after training or before bed in the off-days. Sweat seems to have impact on my bacne/shoulders acne, it cloge pores and could be a culprit for the heat rashes. Also - sleep with a t-shirt which you change daily.
Also definitely try Zin (30-50mg) and Vit C.