Assume they open a Multi 2♦ which is standard 3 way (weak 2 in Major, strong NT and strong 4-4-4-1 or at least not a mini Multi). The assumption would be that this will mostly be a weak 2 and if it's a strong option the likelihood of holding an opening hand is very low (at some point this won't hold and we could find ourselves declaring something awful and doubled, but on occasions that's a fair price to pay). Therefore the multi becomes:
- X = Multi
- 6 card Major - opening hand
- NT opener - 20-22 - something to stop Majors
- 4-4-4-1 hand 16+ (could be 5-4-4-0)
- 6 card Major - opening hand
- 2♥/ ♠ - 5 in the bid Major and 4+ in unspecified minor - opening hand
- 2NT - 5-5 in the minors - opening hand
So basically turned the weak options into stronger ones.
Next assumption - LHO will bid 2♥ so our continuations barely change
- pass - I would have bid 2H - nothing much to say - 0-9?
- 2NT - asking bid - 9+
- X - I have hearts - I haven't thought this through vs showing shortness in hearts, but for ease of remembering we use doubles of artificial bids as lead directing / showing that suit so this is simple.
LHO could pass so more bids available
- pass - I have diamonds and values
- 2♥ - not much to say pass or correct
- 2♠ - invitational in hearts
- 2NT - asking 9+
Against a Multi 2♣ (e.g. showing an Acol strong 2 in any suit, weak 2 in diamonds, a strong balanced hand) the odds change slightly but I imagine the weak 2 is still most likely (assuming the strong balanced is top end. So I think the only things that change are:
- You bid 2♦ for the Multi
- LHO might double and partner mustn't forget they probably have a weak 2 in ♦ if passing
This also leaves other options open - e.g. Leaping Michaels and delayed bidding options where you have a takeout of spades (or diamonds in the latter case) that doesn't much the hand patterns - you know you'll gat a chance for a takeout double when the bidding comes back around. There are still some hand patterns missing (e.g. weak and strong NT hands, takeout of hearts) but I'd rather miss these out to get the bidding right than include them and get the bidding wrong.