Effectively you are setting the default for P3 to be an empty string.
Now that's some good lateral thinking and will probably resolve my issue 😀
the report will go ahead and run itself
Gah, I hate that behaviour. If it was a property I could turn on and off it'd be a wonderful feature but without me being able to control it it's just a pain. On the whole BIDS is a very good package, particularly given that it's free, but there are one or two design decisions which seem... odd.
Oddly enough I've just experienced a very similar problem on another report. It had a bunch of parameters and I made one of them obligatory (ie no nulls). All the subsequent parms that were based on queries immediately became disabled and would only enable when the first parm was selected and I noticed the parms were refreshing at that point even though their query didn't contain a dependancy. I've got a working theory now. I think sometimes the dependancy chain just get's too complex for SSRS to keep track of and, at that point, it defaults to a state of "better safe than sorry". It disables everything it's not sure about and offers them up to the user one at a time. Of course, I'll never be able to prove that but it'll do for me.
Thanks again for your help on this.