For Falc skills :
Pheasant atleast 13, you can max it (more damage at the cost of more SP, especially with attributes). Aiming is a must-have, same for Pre-Emptive Strike
Since you’re using Pre-Emptive Strike, you should max Sonic Strike (more damage), after that, get Roost (huge damage boost for Falc skills in general, if you’re a Kneeling Shot user it gets even better if you do Roost > Pheasant > Kneeling Shot, it’ll also boost every Sonic Strike that’s triggered through Pre-Emptive Strike).
For Call, iirc it’s needed because otherwise you cannot learn the attribute to make the falcon stay near you. It’s also useful in case the falcon AI bugs (wich rarely happens once you get the attribute). If the AI bugs, just use Call and everything will be fine again.
Circling is also a must-have, even at lower ranks.
Personnaly, i have it at lvl 8 because i maxed Pheasant, and it’s usually good enough.
However, it all depends if you want more damage with Pheasant, or if you want to be a bit more supportive with Circling.
Now, for Mergen and BM…
BM is popular, yes, and with Silver Bullet it’s pretty good for a lot of high-level content (and, well, most of the new content) because there’s a lot of Dark property mobs. BM is also pretty good for AA builds.
Mergen, i don’t have one, but from what i’ve read it’s also pretty good, could be better than BM for large group of mobs. Also since Mergen uses Bows, it means you can just focus on 1 weapon instead of 2 (BM has to focus on 2 : Gun for BM skills, and Crossbow for Falc skills & damage). Mergen is definitly less expensive.
As for attributes, there are a lot of important ones to get on all 3 classes, i doubt 3K is enough just for the effect ones.