Looking at the specification, it looks like the relays are designed for
   signal isolation, and the 5V is provided as the control voltage, not
   the actual load
   voltage. [1]https://web.archive.org/web/20181125095912/http://modtronix
   .com/prod/mod/mod-rly2-5v_schematics.jpg
   It would actually be workable to put 3 in series. I'm not sure how to
   tell if it's needed. I guess I'd look up the relays themselves. I'm
   suspecting a single one might handle 12V given how big they are.

References

   1. https://web.archive.org/web/20181125095912/http://modtronix.com/prod/mod/mod-rly2-5v_schematics.jpg