I tried pull-down GPIO15 with 10k resistor. Booting message was disabled.
Serial.Print() function is still working fine.
Problem is solved.

I'm connecting 3x esp32 devkit1 together. Serial1 and other pins already is used for other work. Serial0 connecting together via RS485. But every time boot esp32 sent booting message sometime make other esp32 hang. I searched on forum: pull-down GPIO15 can disable booting message but also disable Se...