Simple print parts with normal PLA , the STL files are orientated for printing without support, you may need to slow the printer down whilst the last part of the support beams are being printed.
Glue the three (red) support beams to the front (blue) body.
Add wire (paper clip wire) between the eyelets of the three arms.
I highly recommend you watch the above videos for details of actuator build.
More build details can be seen here :- https://hackaday.io/project/1011-trimaxgripper-linear-servo-actuator-gm
Fit in the linear Servo mechanism (i used a Pritt-stick as body and a lip gloss tube as the plunger (glue the plunger insert spigot inside the lip gloss tube) and glue the plunger crown (green) to the top of the lipgloss tube). It is done this way so that you can fine tune (sliding the crown up/down) to bring the gripper ends together to give max. grip
The Servo is a standard HS-311 - when converted to a linear actuator it can push 3kg of force.