Me and my boyfriend want a triad but I’m worried about the possibility of him getting another girl pregnant (obviously not on purpose) I need advice what’s a solution around this so its insured it never happens

There are some things in life that you can’t ever be 100% insured against - and unplanned pregnancy between two sexually active people biologically capable of creating a pregnancy is one of them. 

You can take steps to avoid this, like making sure your partner always uses a condom and/or only has sex with women who use some form of birth control. But condoms and birth control can fail, so you, your partner, and his other partners should discuss this ahead of time. Definitely have Plan B on hand in case of a condom breaking. If he never plans to have kids, he could get a vasectomy (but even those can fail!). Be aware that no matter how careful everyone is, there is no completely guaranteed way to prevent an unplanned pregnancy in this situation.

If you don’t trust your partner to wear condoms 100% of the time, that is an issue that needs to be addressed before you two take any further steps. If he doesn’t trust a woman to be responsible and honest about her birth control use, he should not have sex with her. If you don’t trust him to hold to these boundaries, or if he won’t agree to them, you two are not ready for a poly relationship.

Your partner should also discuss this with his partners before having sex - if his expectation is that an unplanned pregnancy would be terminated, and a woman he sleeps with is less rigidly committed to that, they both deserve the right to make an informed decision about whether to risk pregnancy with someone who doesn’t share their plan.

There is no solution to “insure” that this “never” happens, unfortunately. The best you can do is to require honesty and responsibility from all parties. If you cannot live with a poly arrangement that carries a risk of pregnancy, however infinitesimal - if another woman’s pregnancy by your partner would be an absolute dealbreaker regardless of the outcome - then you cannot date a man who has sex with other women.

Side note: I don’t have enough context from your letter, but you may need to rethink some of your conception of polyamory in your situation. You say you want a “triad,” which usually means that all three people are dating each other in an equally committed way. If you’re worried that he will get another woman pregnant because you feel that should be something only you and he do together, or you otherwise worry it would threaten the primacy of your relationship, a “triad” may not be what you’re looking for.

If you’re just worried about any pregnancy in general, because you and he don’t want to be responsible for a child, and you would be just as upset if he got you pregnant - if there is no emotional weight to the worry of “him getting someone else pregnant” and it’s just “a child is created via a relationship I’m involved in,” that’s different. Think hard about what you’re expecting from, and offering to, any woman you’d be including in your relationship so you can be clear with her and with yourselves.