How do I work out how much modelling chocolate to use for a large three tiered wedding cake wrapped around in the chocolate?

Are you only wanting to wrap the sides in the chocolate? I'm not quite understanding just what you want to do.

To wrap just the sides, do some math:

#1 Measure the circumference of all pans required for this cake (using a sewing tape measure) adding up the measurement of each pan

#2 Measure the circumference of a cake pan for which you have already made such a cake and therefore you know how much modelling chocolate you used. Call this amount "one batch".

Divide the big total #1 by #2 and that tells you how many "batches" of modelling chocolate you need. Add 20% for insurance.