I need to level up my gt3 rs? How can i get rest of blueprints??

the gt3rs was a special event car, so now that the event has finished bps will only be available as a possible reward from special purchase crates or they will be added as a reward to replayable races for a short period of time, but theres no way of knowing when they'll be added or for how long, so dont hold your breath that you'll be completing the car anytime soon

Happy to have been of help to you.
If you want to know in advance the events, boxes and rewritable, 
you can contact me in pvt, i do not think it is by regulation to put it public.