I'm a do it yourself homeowner. I ran three separate low voltage lines to some uplights for my palms. Each line has 3 uplights. I have all of the lines returning to the outdoor GFCI on the house. The cable I used is low voltage 12/2. The transformer I am using is a Intermatic ML300RTW. I am having trouble connecting the cables to the screws at the bottom of the transformer. I twisted each of the 3 stranded wires together but the bunch of them is really thick and I am having trouble bending them around the screws. Can I use a pigtail for this application? Or should I be trying to connect the wires without bending them into a curve around the screw?

I would use a properly protected pigtail.

If the conductors will be exposed, they must be placed in a waterproof junction box.

Pigtail (unless you don't have room in the box)