In today's dating rules, you're not exclusive after three dates unless you've discussed it, so he hasn't technically done anything wrong. However, it does mean that after three dates, he isn't thinking of you all the time. If there was potential for something good, you'd be the only woman on his mind right now. Instead, you're just one of many options - and there are easier options out there.
Most importantly, why is he explicitly telling you he's sleeping with other people?
To pressure you into sleeping with him too, so he stops looking elsewhere?
To get you to be the cool girl and say that's fine, so he has your blessing to sleep with even more random one night stands during the period of time in which he's supposed to be getting to know you better?
To make you feel like you'll never be his first choice? Because that's not at all how an unhealthy relationship starts...
To ease his own guilt?
Although I'm all for honesty, I feel like there's no good reason why he's been honest with you three dates in. Everyone dating these days knows there's a chance that the person they're casually seeing is casually seeing other people - actually talking about it seems to have some underhand motives attached.