Specifically in the CO and BN? Should we still be using the same ranges as if we were in the same position as if the table was 5-6 or should we be opening even wider as now the blinds come around a lot faster?

I've been using the same ranges (~60%), but have been considering opening wider. Actually if the SB has a high fold to a PFR I've been looking at treating this situation as essentially a HU match (acting first PF and then having position post) and thus open raise from the BN ~80%.

Generally, your ranges should not depend on how many players are dealt in, only on your position and the position of the player(s) acting in front of you. So if you are in a 9-handed game and it is folded to you on the button, your range should generally be the same as if you are playing 3-handed and on the button. There are some marginal card removal effects when a bunch of players fold to you (it suggests the "good" cards might be "bunched" in the deck and the remaining hands), but I think the impact is far too marginal to act upon.

In some cases, your opponents will have different ranges depending on how many players were dealt in, notwithstanding the above. For example, some players will play too loose when it is short, and so you will need to take that into consideration.

The rate at which the blinds come around is irrelevant. Every hand is independent. Your ranges are based on what hands constitute an (optimal/maximally exploitative) strategy based upon your position, the positions of the other players, and their ranges, not on how quickly the blinds move. Even in a three-handed game, you should make plenty of money on the button to compensate for what you lose in the blinds.

For a long time this had been the conventional wisdom, but the advent of solvers has shown it to be somewhat wrong.

Opening range on the BTN in a 3-handed game should open a range that is roughly 10% larger than BTN in a full-ring game should open. E.g. if the full ring BTN is opening (number pulled from ass) 50% of all combos, the 3-handed BTN should open 55%.

The difference is going to be on the margins, so the overall EV difference between the two ranges is not large at all. If you have preflop ranges that you like, you are not giving up much EV at all by using the same ranges for short games as for full ring.

^^^ Interesting. Did not know this.

When I play 3-handed my opening range is enormous, anything except absolute trash. IMO, if a player in a 3-handed game is not willing to do this -AND bet when checked to even when missing the flop -, that player should not be playing 3-handed.

There will be enormous swings but at 3-handed a player MUST be really aggressive or get crushed by the blinds. Waiting for premiums is out of the question.

Idk I still think that having some checkbacks is still important in 3 handed spots. Cbetting 100% on a 789sss board doesn’t seem appealing to me no matter how many handed are dealt in.

Having a wider range preflop on the button is good, but don’t neglect having a solid sb 3betting strategy. We want to be 3betting a healthy range after the button opens. Apparently there are even hands that fit into a sb calling strat but I don’t know what they are.

OP asked specifically about CO and Button. I have a wide 3-betting SB and BB range as well, fwiw, plus I suppose that I should have mentioned that checking the flop is OK if it's really horrible for us.