Hire road bike for one day for race (never ridden one before)?
In your first tri , you’ll have all sorts of new things to concentrate on, I don’t think an unusual bike should be one of them. 

When I’ve seen people ride road bikes for the first time, it’s the gear shifting that’s often trickier and more distracting than anything else. I’ve seen people take more than one ride to get the muscle memory to change gear in the direction they intended. It becomes intuitive, but through practise. 

For your first tri you’re also only racing against yourself. So if you know how fast you normally ride in training, you can compare how fast you race, and know if you’ve done a good job on race day. 

If, afterwards, you want to do a second triathlon, put the rental fee you save towards a road bike to train and race on.