Based off the pre-flop sims I have (which allow for multiple open sizes, 3B sizes and flats from all positions), 89s is a low frequency call in this spot (roughly 15%), with us folding most of the time (about 55%), and 3Bing to between 6 and 7.5BB the rest of the time (about 30%).
As such, the call pre-flop is OK, assuming it's not your main play here.
When we get to the flop, if we are multi-way on pretty much all board textures, from the limited amount of multi-way post-flop sims we have available (MonkerSolver and Simple 3-way, as well as PokerSnowie), the favoured sizings tend to be around 1/4 pot. The reason for this is that the burden of defending against bets is shared amongst all of the other players, and as such they can fold much more often without being exploited. As such, a sizing of either 5 or 10 on the flop would be better. By betting 20, our opponents are able to choose to only continue with strong hands/draws without being exploited.
When we get raised on this board, I'm not too familiar with solver-based flop 3B strategies multi-way, however if I were to guess I'd imagine solvers would be mixing this holding between calling and 3Bing at some frequency on this board. Due to the highly dynamic nature of the board, having board coverage on a variety of runouts is quite important, and as such there will be very few (if any) hands that are pure 3Bs on the flop.
If we assign the BB the range from the same pre-flop, they will have a lot of flushdraws, A2s-A9s, K2s-K9s, Q3s-Q9s, and all suited connectors, 1 gappers and 2 gappers from 24s to 96s. As such they will have a load of flushdraws in their range, all of which should raise the flop at some frequency (exact frequencies will be combo dependent).
It's worth noting that no suited broadways should be in the BB range, as those should be 3B pre at a pretty much 100% frequency facing this action. They will also have 22-99 at a very high frequency here for all sets, as well as all 98s. They have a very small % of 84s and 98o for more 2P combos, but we block those quite heavily.
The other hands which are in BB's range that would be presumable check-raises here are JTo, QTo and QJo, all of which are in the pre-flop range at a high frequency in this configuration.
Given this breakdown of the BB check-raising range, this turn connects with basically all of their bluffs, and shifts the equities quite significantly into their favour. Additionally, our pre-flop range is already quite condensed, and we should have combos of all sets, combos of all JTs straights and A8s-AJs, KTs and JTs flushes. Besides the T9s-A9s top-pair hands, 98s is near the bottom of our flop calling range on this turn, and as such we should be able to fold pretty comfortably here.
I've attached images from the sims of the pre-flop ranges with frequencies for both hero and BB to assist.
BB range:
Hero range: