Responding to partner's opening 1NT with a 3-level suit bid is a slam try.
Opener rebids 3NT with no interest, otherwise he/she treats the 3-level suit bid as a RKCB asking bid. In responding as RKCB, the next bid up shows 1/4, two up shows 0/3, etc. 3NT doesn't figure in this stepped sequence of responses as this is reserved to show no interest.
e.g 1NT P 3♥ P
3NT
shows no interest, whereas
1NT P 3♥ P
3♠
shows interest with 1 or 4 keycards
1NT P 3♥ P
4♣
shows interest with 0 or 3 keycards. 4♦ shows 2 keycards without the queen, 4♥ shows 2 with the queen
After opener has made one of the ambiguous rebids, 1/4 or 0/3, responder can now bid four of his suit as an enquiry. In this example, 4♥ says pass with zero or 1 keycards, bid on with 4 [somewhat unlikely] or 3.