Not possible while using the builtin ReplaceS function (the parameters are fixed, not parsed for dynamic content), but doable with a custom script.
With a custom script, named ReplaceStr, with this code:
function ReplaceStr() {
var params = CurrentField.value.split(',', 3);
if (params.length == 3) {
return params[0].replace(params[1], params[2]);
}
}
you can then use this syntax: [ReplaceStr](source,searchValue,newValue)
With your example: [ReplaceStr]([F],replace this,[myScript])