There are two fees in here. One is 3 sat/B and another is 5 sat/B. The 5 sat/B is what I set in Electrum but what is this 3 sat/B?
Pay attention to the unit. It's 5 sat/
vByte and 3 sat/Byte. The former is related to the new unit representing the block sizes after Segwit, which is called weight unit.
In your case, this tx has a size of 236 Bytes and 142 vBytes.
710 satoshis/142 vBytes = 5 sat/vByte
710 satoshis/236 Bytes = 3 sat/Byte
If you send coins from a Segwit address, your tx actually weights less, so your sat/vByte rate is higher than if you used a legacy address with no Segwit. That's why you have a discount on the fees.