Perhaps this is what you're looking for. I created 3 new variables of type Double (Cutoff 1, 2, and 3) and then used the Collect Values tool (found in the Utilities menu in ModelBuilder) to collect these into a multivalue of doubles. The output of Collect Values can then be connected to the Cutoffs parameter of make service area layer.
(Just to be clear, I renamed the output of Collect Values from the default to 'Cutoffs'
To create those 3 double variables, right-click on blank space in your model and chose 'Variable' (Ctrl+E). You'll get this dialog:
There are a LOT of different variable types you can create -- it's kinda interesting to scroll thru. But I digress -- press the 'd' key a bunch of times and eventually 'Double' will show up.