What you have is a microcontorller-based OpenSprinkler (looks like it’ a DIY kit), it’s not OSPi. So this is actually not the right subforum for this post.
The A3, VIN and GND are the three pins to connect to RF transmitter (they correspond to DATA, VCC and GND). I apologize that the user manual is only showing an exmaple for the fully assembled OpenSprinkler, while yours is a DIY kit. But yes, A3, VIN and GND are the three pins you are looking for.