If he's telling you has done a poo he is aware of his bodily functions and Just needs to know where to put it.
Buy the book "Oh crap" it's brilliant, don't wait for signs, show him the way and dignity to use the potty.
You may throw his toy storage potty and buy a different one, so he doesn't associate it with his toys. This is not a toy, it's where his sees and poos go.
Kept him home for a week, clear diary (easy enough at three moment 😂) it's about reaching stages not days or time limits.
Strip him off completely, watch for his cue (no phones, no tv for you, watching your kid is what you are doing) he will have a cue before he has a wee or poo. Put him on the potty to start with when he stars to see or poo. Might take 2 days to see his cue, he will have one .
Once he knows Where it goes and uses the potty more often then not by himself, you Must hand the control over to him.
Introduce his trousers, baggy trousers (I'm assuming he can pull his up and push them down as this is needed for training )
Don't use underpants for about 4 weeks just trousers.
Then start a short walk, have a wee before he goes, don't force him onto the potty ,just tell him you are not going out until he has tried for a wee, and mean it.
Once you arrive at a destination he goes the toilet, if he says no, you just accept that he doesn't I keep
Then start with naps and bed fine or do them together, the book tells you it all
Quick run down, I've probably missed lots, but honestly buy the book before trying other ideas, it's a game changer. Our son was going on his own at 21 months and dry at night 2 weeks later.....game changer!!