For a three phase sofstarter you need the following:
Control section:
I have one sample program for an 8051 derivative so please pm me.
Power section:
1.- For each phase you need two SCR (or thyristor as you say) connected back to back so that they function as a controlled switch for the AC current/voltage.
2.- Depending on the main voltage, you need to protect the SCR using and Snubber Network, basically composed by an R-C arrangement along a varistor, please check the powerex or IR web pages for example.
3.- For firing the SCR and also for isolating the power section from the control one you have to choose Optoisolators or Pulse transformers along their associated resistors, diodes and capacitors.
I have a basic arrangement available, so anyone interested please pm.
4.- It is suggested to install one fast acting fuse (for semiconductor use) at each phase input.