You need a quad box. But I don't know of any HDMI IN versions. I have a Decimator MD Quad http://www.decimator.com/Products/MD_Quad/MD_Quad.html, but the only HDMI connection is an OUT(it has four SDI IN and an SDI and HDMI OUT). There are HDMI-to-SDI conversion boxes out there, too, but you're going to get into some serious money between three conversion boxes AND a quad box(the MD QUAD is $1200-$1300 alone).
You will definitely need to convert the signals to SDI. This is a smart thing to do anyway as your three cameras will likely be a decent distance apart and the cables will have to run a long distance to be positioned to avoid getting underfoot of the audience. SDI cables are far more robust and a better bet over any real distance. There are HDMI-SDI converters available for very low cost these days, sometimes under $100 if you look on Amazon.
The MD Quad is a good product for this. Be aware that it can introduce a minor delay into the signals. There is also the product from my employer, the Convergent Design Odyssey7Q. The Odyssey7Q can take up to four 1080 HD-SDI inputs at once and display them all in a quad-divided image both on it's built-in 7.7" OLED screen as well as outputting to two SDI outputs and an HDMI output for additional monitoring.
If you just need this for a single concert you may want to look into renting the gear rather than purchasing.