No idea how to do this - help requested

I have a problem with a conference registration form which I'm not sure how to overcome. I've set up many of these for a client, using a form on an HTML page which then posts to an ASP page which emails the form data to the client as an email, then redirects to a thanks page. So far, so straightforward. This time round, the client has decided he wants something different - the plan is to run 3 conferences next year, and depending on whether the viewer selects more than one conference, a different level of discount applies.
So, for example, if you book one conference, this needs to trigger a message (presumably a hidden field) saying: You've selected 1 conference at a cost of £xx.
Booking two conferences (either 1+2, 1+3 or 2+3) should trigger a message saying:
You've selected 2 conferences at a cost of £yy - that's a saving of £zz (a bb% discount)
Booking all three conferences should trigger a message saying:
You've selected all 3 conferences at a cost of £zz - that's a saving of £xy (a cc% discount).

I'd thought of trying to do this with javascript but have been persuaded that it won't be much use if the viewer has his/her javascript disabled in the browser. If it was just a choice of 1, 2 or 3 checkboxes I assume it would be fairly straightforward, but as there are 3 permutations for choosing two conferences, I'm a bit flummoxed.
If anyone has any idea of a simple way of achieving this, or has done something similar, I'd really love to hear about it. Thanks in advance!