I have a battery, that supplies 7.5V but the raspberry pi 3 can only handle up to 5V in the micro usb input. What would be the easiest way to handle this? I have a plus and a minus cable and no real way to modify the battery. It has to be something like a bridge.
You need to buy a drop down converter.
I would suggest a UBEC (Ultimate Battery Eliminator Circuit). These convert a range of input voltages (typically 6V-26V) to a fixed 5V. Many are rated for 3 amps which is more than enough for the Pi. UBECs are typically used by model aircraft hobbyists to power servos and other on-board electronics.
You can pick one up for a few GBP or dollars on eBay and Amazon.