When we hold 4 card support for partner's 1 of a Major (1M) response, we simply must find an appropriate bid to show our partner about our fit and relative playing strength. Many people have different opinions on raising your partner with only 3 card support. The problem with raising a 1 level major suit response with only 3 cards is that partner may only have a 4 card suit, leaving you in a 7 card fit.
IMPORTANT NOTE: Never jump raise your partner with only 3 card support, find another bid
I use the following criteria when deciding to raise with 3:
1) I think 2 of our Major will be a significantly better part score than my alternative rebid.
2) I want to encourage my partner by showing a fit. (Partner will be more inclined to push for game with a fit)
I like to raise partner with the following hand...
Here I have nice playing strength for ♠ and have a soft spot for NT with such weak ♥. I also believe it will be harder for my opponents to compete in ♥ if I raise directly. If partner pushes to invite, my side suit of ♣ will provide extra tricks. Often one of the reasons that partner will aggressively try for game is because they think they have a double fit.
If you and your partner agree that your style is to commonly raise on 3 card support, it is great to have a convention to sort out what kind of raise you have.
There are many different versions of this convention, this is a modified version that my partner Vincent Demuy and I came up with that we found effective.
1m - 1♥
2♥ - 2♠* Asks partner what kind of raise do you have? then...
2N = Balanced 3 card raise, minimum
3♣ = Balanced 4 card raise, any strength
3♦ = unbalanced 3 card raise, shortness in other minor
3♥ = unbalanced 3 card raise, shortness in other major
3♠ = Splinter, 4 card support
3N = Balanced 3 card raise, maximum
4om = splinter
4m = 5422 MAX
4♥ = 5422 MIN
1m - 1♠
2♠ - 2NT = same
3♣ = BAL 3♠ MIN
3♦ = BAL 4♠ ANY
3♥ = UNBAL 3♠ shortness in other minor
3♠ = UNBAL 3♠ shortness in other major
3N = BAL 3S MAX
4x = SPL
4m = 5422 MAX
4M = 5422 MIN
This system can also be used after 1♥-1♠-2♠.
NOTE: After BAL 3M MIN, going back to 3 of partners minor is NF
NOTE: After any response, 3M is NF
Also it is good to play 1x-1♥-2♥-2NT = 4♠ 4♥ forcing... so you can catch your spade fit when partner has raised on 3 card support.
In our history, we found most of our good boards came after the response which showed 4 card support and a balanced hand. After this bid, each partner gets a vote on whether playing 3NT was better than playing our 4-4 major suit fit. The biggest reason for this to be the case is because we both have weak trumps. Responder could choose to bid 3NT directly over the 4 BAL bid to express maximum interest in playing 3NT. They could however choose another bid below 3NT allowing partner to suggest 3NT knowing that responder didn't have the MOST extreme preference for 3NT.
I am sure over the years many people have invented their own pet versions of this convention. Please post them here and describe what situations you like best about your methods.
Plus... it's free!