Perhaps you want to use an input parameter Data Type of Layer File, and set its MultiValue property to Yes. Your tool's dialog will then allow you to select multiple layer files and add them to a list.
The Parameters tab of the Properties page of your script tool is where you specify the Data Type of your input parameters. You'll notice that clicking in the Data Type column for a parameter gives you a combobox of the available GP datatypes. And if you want to read more, see this PDF.
A boolean input parameter gets rendered as a check box in the tool dialog, so that's probably inconvenient.
In your case you specifically want to pick multiple layer files, so you could set your input parameter Data Type to Layer File, and MultiValue property to Yes.
And if you don't want to bother your user to type or browse to the layer files, but those paths are known ahead of time, you could set the parameter's Default property to be a semicolon-delimited list of catalog paths to the layer files "c:\temp\layerA.lyr;c:\temp\layerB.lyr", which will pre-populate the layer list shown in the tool's dialog, and the users could remove the ones they don't want to use?
the tool dialog will look like (nevermind the error my example doesn't point to actual data on my system so it fails validation):
Perhaps getting even closer to your original goal, you could use a parameter of type String instead of Layer File and provide a Default being a list of aliases "Layer A;Layer B;Layer C" in this way, and have your script do the work of translating each layer alias to its catalog path. The dialog looks like this:
blah blah somebody stop me.