On the L1 Pro series combo jacks, the XLR connections are for mic level signals, while the 1/4" centers of the combo jacks are for line level signals when using TRS balanced cables. The 1/4" jack on channel 3 is also balanced line level. (The 1/8" jack on channel 3 is for unbalanced stereo mini plugs, such as when connecting to a phone, portable music player, or laptop.) So, using your XLR cables would likely overdrive the mic preamps on channels 1 & 2.
The easiest option is to use 1/4" TRS to 1/4" TRS cables to channels 1 & 2:
This will provide a balanced signal, with an added advantage: When you're setting up, you don't have to worry about which end is male or female!
If you really want to use your existing XLR cables, you'd need either a couple of short XLR female to 1/4" TRS patch cables, like this:
or XLR female to 1/4" TRS adapters like this:
The short cable is preferred, as the adapter/XLR combination becomes large and clunky when sticking out the rear of the L1 Pro and would be prone to breaking off if bumped.
I made a mistake in my previous reply, and I have edited the reply and pictures to reflect the correct answers.
If you use XLR cables, you'll need XLR female to 1/4" TRS cables or adapters. They go on the ends that plug into the Pro16. (I mistakenly wrote "XLR male" earlier.)
Thanks for taking the time to respond. However, upon consulting with a Bose Tech, he informed me that all 3 inputs can handle line level signals XLR or TRS. Just have to make sure on inputs 1 & 2 the Tonematch is set to off and not on MIC or INST. Just 3 can be used as is with TRS 1/8" or 1/4".