The only problem with "3-Day Potty Training " is the name, because it implies it will be short and easy, and it rarely is ; ) My grandson trained at 32 months in 4 days, day and night training completely (no accidents) but Mom had been talking about it and letting him see her and dad use the toilet for 8 months!
And I agree with V K., the longer we wait to train past a certain age, the more set in their ways our children become, and the harder it is to train them :( I didn't act on my guy's interest at 21 months, and I can now see I missed my window of opportunity so it's a little slower going. I've also read that girls are typically ready physically to train at 22 mos, boys about 3 months later at 25 mos. And, prior to disposable diapers and pull-ups the average age a child was trained (not starting) was 18 months.
The goal of "3-Day Potty Training" is to teach your child to recognize the urge to use the bathroom, and it sounds as if your son did that since he was going solo on his own. That said, I would take a total break from the potty, leave it out and in sight but don't mention it for a week or so, then re-group and start again.
When you begin again I would leave him bare-bottomed as much as possible so he has easy-access to the potty, not use pull-ups but cloth training pants and plastic pants over them (I bought both at Target in the baby/toddler section, they were next to regular underwear) so he really feels the wetness and yuckiness next to his body, have him clean up the messes and change any messy clothing, stop rewards and sticker charts (see my personal exception below), wait until he's mastered PT 101 at home before suggesting he use a public restroom, and get a potty seat that goes on the toilet for encouraging him to use the big potty when you begin that, along with a stool to make it easier for him to get up there.
1) Set regular times to tell him (don't ask) it's time to use the potty. With my guy (27 months) it's when he wakes up in the morning or from naps, before going to sleep either time, and after meals or drinking. The rest of the time he can use it on his own, which he sometimes does, and I will suggest it if I notice any signs.
2) Encourage him profusely for successes and trying, give him a "high-five" and clap. (I go all out clapping and praising him and he loves it. He stopped asking for stickers, which he'd wear a few minutes and then forget about, and I discovered that Dum Dum pops are a great incentive to get him to poop when he resists and is trying to hold it in. I'll give him one each time he poops until he's doing it regularly on his own and wean him off them.)
3) Hold off putting him in regular underwear until he's going regularly on his own, use it as a "big boy" incentive.
4) Show him how to rinse and wash out his pants in the sink, Zote is a pink laundry soap that works well, then the next time and so on he does it on his own. This worked with my nephew, he never had any accidents after he washed his out the first time.
5) It sounds like you are doing great, and I know it's draining :-/ But hang in there!