Are all 5 bolt chainrings the same 'fit', i.e.spacing between bolts (BCD?)
Or, am I right in thinking there are compact and ATB and they're different?
Looking to replace 26/36/46 on a tandem.

Anyone, please?

yours sound like 110mm bcd, microdrive rarely goes over 42t big ring.

There's a nice chart of bolt-to-bolt distances here for easily checking exactly what you've got:
http://www.sheldonbrown.com/gloss_bo-z.html

But, as said, with those rings it'd be more likely to be 110mm BCD (ATB) rather than 94mm BCD (compact triple/microdrive)