Last night, I put the gear down early on the approach because of the need for additional drag. At some point during flap extension, I have experienced this issue a few times. Wondering what I am doing wrong? Thanks,

Uh, what exact issue? Are you saying you had the horn with three greens at the same time? Because then it would probably be an issue, otherwise not, obviously. Once you extend flaps into landing range with gear not down and locked you should hear the horn, but obviously not with the gear correctly in place. Otherwise, your issue could very well be of a completely different nature just as well... So first of all you should definately check and make sure you set up your pressurization like a pro.

Have you exceeded gear extension speeds?

Probably is because you should extend the gear before sting flaps 15