At 3.5 weeks I'd hardly be even trying to bother with the stubborn ones - save yourself and the snakes the stress and just let them get hungry rather than establish an unpleasant behavioural pattern of refusing feed - if it's always there and you're in the habit of knocking it back, it's not helping anyone. If it rarely comes along you're more likely to seize the opportunity.
Other than all the usual tricks (scenting, assist feeding, braining, leaving in unattended, etc etc etc) if anyone has any secret techniques, they're a very well kept secret which after 30 years of getting baby snakes feeding I haven't found (unless I'm also hoarding the secret).
I very rarely bother with scenting etc, and mostly it comes down to learning a good technique with jiggling the pinky around in the right way, which comes with practise and experience, which given it's your first time you'll be at the most challenging time with, but the good news is that it just gets easier from here.
As for when to panic? Never. Panic never helps anything. As for when to say 'Okay, if they're not feeding voluntarily I'd better use more convincing techniques such as scenting (as I said, I don't use it, but it's an easy one for newbies to use and back in the 90s it was a go to method for me), assist feeding, force feeding etc, I don't at all go on time, I go on when the snake is starting to lose condition and needing it, which can be anything from a week or two in rare cases when the snake hasn't properly absorbed the yolk, through to several months. Usually by about six weeks I'll be thinking about assist feeding the few which haven't yet fed. At under a month it's rare that I wouldn't just say 'Okay, I'll offer you a pinky next week' and I wont try very hard at all - I'd rather they decide to voluntarily eat at six weeks than I force something down their throat at four or five.
Judging when they're starting to lose condition is a challenge for some people and you don't want to leave it too late - they do get to a point where they've lost enough condition that a feed will actually kill them, and once they're at that stage there's nothing you can do, but wait until they start to look a little skinnier than ideal before being concerned, then at that point just do what's necessary to put a mouse into the snake. Even within a single clutch of snakes this could be five weeks for one individual and 3+ months for another.
One thing I strongly recommend is not to try too often - this just stresses both you and the snake out. If something doesn't work, don't try the same thing again within a day or two (that 'continually trying the same thing and expecting different results is insane' line comes to mind). If the snake isn't needing a feed urgently, try again next week and don't go near it in the mean time. If it is urgent, try whatever methods you have in your repertoire with force feeding being the final one if necessary, and get a feed into the snake.
Oh, and I wouldn't bother trying too many little things. For example, I would never heat the tongs (both because it's stupid and because I just wouldn't bother with such trivial details), you're not going to get a better result feeding outside the snake's tub, there's no benefit and a lot of problem with feeding from your fingers rather than tongs, time of day isn't usually that big a deal but early evening is usually best and earlier isn't worth bothering with (by all means try earlier if it's convenient for you, but don't make a special effort to feed during the day as a special extra method to try). The most important method to use is good tease technique, which is just something you have to learn to get good at over many years and clutches. The last resort technique for those stubborn hatchlings is force feeding (which should be fairly rare once you're good at other methods, but may be more commonly needed before you learn how to entice them).
Well done getting a perfect 10/10 hatch rate in your first clutch!