I agree with @SiouxsieSiouxStiletto , try one thing at a time so you know what does or doesn't work. The caveat on that is that I believe omeprazole can take up to a couple of weeks to be fully effective, so you would potentially need to give that one a bit longer. Are you in the UK? I thought it was unusual here to be prescribed omeprazole until you'd tried carobel/reflux formula milk and gaviscon.
In terms of whether reflux is something every baby has and just out grows, virtually every baby will posset a bit and virtually every baby will have periods where they cry inconsolably and you just can't figure out what is wrong, other than that life is overwhelming when you're new at it. DD1 did both of those things, that was fine, she was a baby, we never considered treating her for reflux (and yes, she largely outgrew the inconsolable evening crying after a couple of months).
DD2 was sick large amounts many times a day and screamed in pain for hours. (I assume your baby isn't like this, as you say mild reflux.) Omeprazole was an absolute game-changer for her, different baby within 48 hours (still sick quite a bit, but stopped the pain). She still had the odd 'witching hour', but it was a different cry.
If I were to have DC3 and deciding whether to medicate for reflux, I'd be looking at whether they were gaining weight well, whether or not they were in pain, and whether it was interfering with feeding (aversion, for example) or sleeping. If baby is a 'happy spitter', gaining weight well despite the amount of milk coming back out and not distressed, I'd probably ride it out. DD2 clearly needed medication. In the middle is harder to decide, and I think depends how sustainable things like holding baby upright for 20-30 minutes after every feed are for you, baby, and the rest of the family.