Suppose opponents open a weak two or three bid against you, and your partner overcalls 3NT, do you know how to proceed? I'm not sure there is a common method, but we shall present a sensible, reasonably simple system.
To start with, when they open a three level preempt, what does 3NT show? A stop and a minimum of a 15 count, with a balanced hand, or alternatively with a stop and a good minor suit. With a major suit he would tend to overcall in the major. Bear in mind that normally you should pass - partner has probably already bid your hand for you!
If you think your hand is strong enough or distributional enough to bid on, we suggest the following methods:
4♣ is used as an enquiry, with 4♦/♥/♠being transfers (with the transfer to their suit showing diamonds). Thus
3♥ -3NT-P-4♠ = asking bid, with responses:
4♦ = Hand based on a long diamond suit (then cue bids, with 4NT natural)
4♥ = Hand based on a long club suit
4♠ = 15-17 points, balanced
4NT = 18-19 points, balanced
Higher = Natural five card suits, and 20+ points.
If they open three of a minor, the same applies, except that now there is only one long minor we might wish to show over the 4♣ bid. Thus 4♦ and 4♥ are both used to show that minor, but 4♦ shows above minimum.
When opponents open a weak two, essentially the same system is played, but now partner will bid 2NT with 15-18 or 19. Over 2NT you should play your usual methods over a 2NT opener.
If partner overcalls a weak two with 3NT, showing 19+, you can enquire with 4♣ as above. Now partner shows 19-21 by bidding 4♠ or 22-23 with 4NT.
An alternative system is to double with the stronger balanced hands, reserving the 3NT bid for long minor hands. A disadvantage with this style is that partner frequently responds to the double with a Lebensohl 2NT, and you end up playing 3NT the wrong way up.