DisplayPort is different than HDMI or DVI. DisplayPort is its own unique format. However, there is a DisplayPort option called DualPort that allows HDMI or DVI to be sent out. I'm assuming that's what you have. However, I can't see how that will help unless Joe knows of a HD modulator that will take HDMI or DVI as an input. I don't.
However, what Joe is suggesting is taking the HDMI output and sending it to all of the displays as HDMI. What is the furthest distance from the macbook to one of the displays? If under 25 feet per cable, then you could get an HDMI distribution amp - probably 1x6 or 1x8 - and send the same HDMI signal to each device. You would have to run HDMI cable to each display. I'm guessing somewhere between $100 and $150 for the equipment and cables, if under 25 feet per cable.
If you need over 25 feet for any of the cables, then you are probably looking at converting to Cat 6 and then getting an appropriate distribution amp. It would likely be more than the HDMI cable solution but would work the same.
I realized this morning that your modulator solution wouldn't have worked no matter what we do. Most new flat panels no longer have an analog TV tuner included anymore. So there would be nothing in the TV that would accept a channel 3/4 input.
BTW, High Definition modulators usually start at around $500 and work their way up. Not knowing your budget, I don't know if that would be acceptable. Again, the other problem is how to get the signal into the modulator, even if it were HD (VGA is a possibility after being converted to component).
This is one of those items that used to be simple (and noisy) in the analog standard definition days but is now much more complicated (but looks much better).