If you have a 32 bit machine or you have a 32 bit version of Microsoft Office on a 64 bit machine, you need a 32 bit version of QGIS 3 (currently you should choose 3.10) and QSWAT3. You choose between QSWAT3 and QSWAT3_64 based primarily not on what machine you have but on whether you have a 32 bit version of Microsoft Office, or a 64 bit, or don't have Microsoft Office at all.
Existing users of QSWAT and ArcSWAT can continue to use their current installation of the SWAT Editor and the SWAT executable with QSWAT3 and QSWAT3_64.
QSWAT3 and QSWAT3_64 are 32 bit and 64 bit ports of QSWAT to QGIS 3. Release Notes for QSWAT3 and QSWAT3_64 version 1.1
