The newest version of SynthEdit, Version 1.2, can save 64-bit VST3 Plugins.
This version is still somewhat unstable, so you may experience bugs.
SynthEdit Version 1.2 come in two flavours: 32-bit and 64-bit. The 32-bit version makes VST2 plugins (V2.3). The 64-bit version makes VST3 plugins.
VST2 plugins are more universal and stable. VST3 plugins are somewhat experimental and unstable at present
64-bit SynthEdit requires 64-bit Modules (most existing modules are 32-bit). Any 32-bit module you used with older version of SynthEdit are not 64-bit compatible.