are you planning on re vegging after you find the sex? three nodes is still pretty young. if its fire i would keep cuts off of it until you get tired of it.i didn't do that on the last bag seed i got just because i didn't know what strain it was because it came from my daughter and turned out to be fire. And no cuts to run again i saver that smoke so stretched it out for a little over a year.
What up Big S, Spider has a great answer . You could also just veg the plant for 3-4 weeks, top it often, and then take clones before putting the hopeful mother under 12/12 . If it turns out to be a male, your work would be for nuthin ! Unless you want to collect pollen. But if it's a female, now you have female clones in veg, and the mother to flower out, to see how "fire" she really is ! GL
I have forced sex a few time and if you catch it early you can flip it back to veg with not reveg however very early flower can be tough to spot and flipping back to veg could jump start a hermi so I usually cut clones and force sex on one clone while it roots or flip the mother like Spider and Chubby 441 said.