Serial Communication Between 3 Arduinos Doesn't Work (Daisy-Chain)
Arduino #2 should have two SoftwareSerial instances, one to listen to Arduino #1 and another one to talk to Arduino #3:

// In Arduino #2
SoftwareSerial tiny1(10, 11); // Listen to Arduino #1 on pin 10
SoftwareSerial tiny2(8, 9);   // Talk to Arduino #3 on pin 9


You may not need to wire unused signals (11 and 8).

There is an example here : https://www.arduino.cc/en/Tutorial/TwoPortReceive

Note that what you are trying to do is a bus. Serial in Arduino is RS232 based and that is not a bus but a point to point protocol. You should consider using I²C or SPI.