It would actually be 1), except that it will be somewhat slower than if there were no B or G clients, as the AP has to transmit B and G compatible frames to tell those clients its busy.
You can look through there and linked articles. In particular, it contains the statement: “802.11g hardware is fully backwards compatible with 802.11b hardware and therefore is encumbered with legacy issues that reduce throughput when compared to 802.11a by ~21%.”
Somewhere there is an article that went into more detail about how backwards compatibility works. (I’m pretty sure in the case of 802.11g, that that ~21% penalty goes away when everything is set to G-only. N has similar considerations.)