God I hate this new format that makes it hard to look through others messages in chronological form without scrolling through the lot. I've read OP's and a few others but apologies if I am repeating points made.
So you were dating, presumably online. Had 3 dates with A whom you'd like to keep seeing. No talk of exclusivity. Meanwhile you had a quick fling with B, shagged twice. Fine.
So modern online dating suggests no you weren't cheating.
The whole exclusivity etiquette is pragmatic but if you really hit it off with someone, feelings for have a role from early on. First date, often. He may well be hurt if you tell him that the early stages involved you sleeping with someone else therefore not that excited about meeting you.
You weren't in the wrong, no. It wouldn't endear me to you though and I may feel put off at an early stage. In bleating 'we weren't exclusive ' wouldn't prevent me being out off. It may take some time to decide whether it was a deal-breaker but in all honesty I think I'd just wish you'd kept your gob shut (unless it was someone I knew closely), and things would fizzle.
My point is, it's acceptable, not cheating as there was no agreement. Nothing to declare. Similarly there is no obligation for him to know. You can maintain a clear conscience and get on with the relationship without feeling the need to blurt this out. That would only make you feel better.
It isn't disgraceful or anything like that, you haven't deceived anyone. I would just feel disappointed. A special connection doesn't start with knowing about my DP shagging someone else the week after meeting me even if he can because we haven't agreed not to do so. Might be a complex mix of ego, other emotions and convention coming into play but I urge you to keep this to yourself. The fact it wasn't a one off I think would make it feel worse.