Hello.
I've read your post several times, and I really can't understand what it is you're asking, so I'm going to have to guess.
1. You're in a relationship with someone that isn't based on trust or honesty, and things are now falling apart. There are no solutions to this, except open and honest communication. You can't make this man believe you if he doesn't want to. It sounds to me like he's a lost cause--or at the very least, he's trying to find a reason to disengage from you.
2. Your boyfriend thinks you're having sex with others, due to the fact that your vaginal opening is able to accommodate three fingers. Again, he's trying to disengage from you--and he's misinformed about anatomy. As you say, your vagina is elastic--it's able to accommodate a baby's body passing through it, so three fingers? No big deal.
3. Because you didn't have any visible blood the first time you and your boyfriend had penis-vagina sex, he now believes you did this with someone before him and lied. Again, he's woefully mistaken. Most modern women no longer bleed during first penetration because they're much more active, with sports, bicycle riding, etc. All these break down the hymen tissue gradually, so many women no longer have bleeding, etc. Not to repeat myself, but it sound like he's looking for an excuse to pull away from you.
Time to ask yourself what's going on in this relationship. Is this the person you want to be with? Someone who makes you feel bad about yourself, who is ignorant and doesn't trust you? These are important questions to ask. Best of luck to you. Dr. J