[GraemeJ]

You must have been very lucky finding this thing. I've been searching for years now for something similar. All I can find are mic/line mixers, nothing suitable (and affordable) for 3 or 4 guitars. And I would really like to avoid having to use 3 DI boxes .

Not so much lucky as already having it.  I purchased it in 1998, for a totally different purpose and retired it 10 years later, when it was no longer in use.  It was just sitting on a shelf until 3 years ago, when I dug it out to use for mixing the guitars. 

It's only downside is that it's designed for the domestic market so has a ton of RCA connectors rather than 1/4" jacks.  You have no idea how hard it was to find proper mono to mono adaptors. I must have tried about six suppliers before I found some!  

The DI solution is technically the best, but I see no reason why you should not use any mixer with line inputs. In your place, I'd borrow one and see how it goes.

At $25, one of these might be worth a punt - http://www.sweetwater.com/store/detail/MX400 . At 4.Kohm the imput impedance is low for a guitar, but definitely worth a try.