Separating Biking into Road/Trainer/Spin

OR

You can configure this as 'equipment' - so:

Road Bike X
Trainer Y
Spinner Z

That way it will all show up as 'bike volume' and you can later go in and run a report for totals on the separate equipment types.

All depends on what you are looking for...

That's your call, though it comes up a lot in the forums.  I put mileage down for my trainer (it will break the mileage down for each equipment, just have to make sure you select it when you log it.)

For the spin class, if you like seeing mileage as part of your totals, you can just take your total time in hours and multiply it by your avg MPH - it may not be entirely accurate depending on the spin class, but it will get you in the ballpark if you like to keep track of total bike mileage.