Sorry, it's not possible.
To use a loosely coupled cluster needs special programming and there's a whole bunch of overheads in running clustering that would swamp the diddy small slow processor and slow I/O subsystem on your RPis.
Microprocessor, Raspberry Pi & Arduino Hacker
Mainframe database troubleshooter
2012-18: 1B*5, 2B*2, B+, A+, Z, ZW, 3Bs*3, 3B+
Any DMs sent on Twitter will be answered next month.