I'm not going to lie to you. I find tripod headstand far more challenging than supported headstand, but everyone's different - my mom (who is in her 50s) can pop right up into this like it ain't no big thang! I've had students in class able to get into one version of headstand and really struggle with the other, so I encourage you to practice each variation and just see where your strengths lie and where there are areas for improvement.
The trick with tripod headstand is to firm up the arms. Make them so strong that if someone pushed them, they wouldn't budge. When the arms are firm, we activate the shoulder strength (particularly, the area where our arms connect to the armpits). I've created a little video below demonstrating how to get into tripod headstand.
Give it a go and let me know how it goes in the comments below!