You remove the JD-VCC to VCC jumper on the relay board and put +5VDC on the JD-VCC pin for 5V relays. Your relay board VCC then goes to 3.3V VCC of your micro. If your 3.3V I/O can triggger the relay via the opto on the board, fine.
If however, the 3.3V I/O does not trigger reliably, you can just jumper the indicator LED on that board to boost the current through the opto.
Putting +5V on the JD-Vcc ensures the relays pull in properly, even though they may trigger at 3.3V. A lower relay voltage may cause the relay to pull in slowly (too slow)