RPI 3B+, booting doesn't initiate

I downloaded and flash the USB flash drive with NextcloudPi_RaspberryPi5_v1.54.0.zip. After connecting to the power supply, the green light turned on and off, the rainbow screen ensued, and after that just plain black screen with no signals from the green led. What can be the probable cause?

They have a different image for Raspberry Pi 4 and 5. There is no dedicated image for RPi3, and you need to use an older image in this case:

I have used a 3B model in the past to do some testing, but it was really slow. Iā€™d rather use a newer model if you can.