Crikey, triplets, can understand your worry! I can empathise too, and want to suggest in the gentlest and kindest way possible that you seek some talking therapy, to offload some of the worry, and also to help deal with this stage of your children's lives, to avoid a heavy bout of empty nest syndrome (or depression, or any other stress related illnesses). You sound worn out with worry, lovely, and I've been there.
If you have any anxiety about your children having children, or the idea of grandchildren, their babies, and so on, related to losing Matthew, then that is another thing to ask for help with. You can see your GP, or self refer online through the NHS.
(I'm now roughly the age you were when you lost Matthew... I am trying to imagine losing my children, then going through IVF now, and having triplets! Oh my word, what you have been through!)
Would your children have been having sex ed lessons and Personal Social Development talks at school? My DD is in the same year group and they've been pretty well educated on contraception. I think if you were to just talk positively about using contraception, sex as a young adult being a matter of choice and consent for the people involved, and perhaps buying some condoms and leaving them in a sock drawer for her to help herself to if needed, it would be a good start. Taking the pill is quite a big committment, and it is good to use condoms. (To know not to mix them with certain gels or lubes etc, too).
I went to our local Pride festival recently, and there were people there giving out condoms and information about sexual health, and willing to approach us and talk about anything. If you have a Pride festival coming up locally, it would be a great place to have a chat about it.