After your hen has been mated, the first egg she lays and maybe also the second one as well, won't be fertilised. If it was me, I'd give it a week before starting to save eggs, to be on the safe side. You won't have to worry about how often 'you' have to mate them, as the roo will no doubt be busy mating them every chance he gets. But do check to see he's actually doing the deed - if he's newly introduced and/or young and the hens don't know him, it might take him a while to work his manly charms on them and get them to submit. If they're not suitably awed by his strutting and crowing, they might well spend a few weeks beating him up and chasing him away. Hen seduction, it can be quite funny to watch...
One roo to three hens is a good enough ratio that every single egg should be fertilised, so you shouldn't have any worries there, but you might find that with only three hens, each of them is being mated so much that they get quite bad feather damage or bald bits on their heads, backs and shoulders from the roo trampling all over them constantly when he mates with them. I keep two roos with 20-30 hens, and I haven't found a non-fertilised egg in over a year. My breeding pens are one roo with 3-4 hens, and at the end of the season the hens look awful. If I had more good hens, I'd want to have maybe 6-8 of them in with each roo to minimise feather damage.
The eggs can be safely stored for at least 7 days before you incubate them, and I've stored eggs for up to three weeks and still had good hatch rates. But if I wasn't deliberately experimenting with stored eggs, 10 days would be the most I'd want to store eggs before putting them in my incubator. Store them pointy end down in an egg carton, keep the carton in a cool place, and tip it from side to side each day to 'turn' the eggs. That maximises their chances of developing properly once you do set them.
When you say 'after the hen has laid a regular egg', do you mean once a pullet has been laying regularly for a while and has stopped laying a weird assortment of huge eggs, tiny eggs, pointy eggs, that sort of thing? It's best not to set a pullet's first eggs, as they do take a while to get their laying mechanism into a regular pattern, and their first few eggs could be double yolkers, no yolkers, thick shells, rubbery shells etc. It's best to wait till they're popping out eggs that are roughly the same size and shape every time they lay.
Hope all that helps you out a bit!
