While there's nothing technically from using your TV as a center channel, I would add my strong recommendation that you don't do it. Many years ago, before center speakers were widely available, I tried it with some AR tower speakers as left/rights and it sounded pretty awful.
Unless you are incredibly lucky, theres not much chance those TV speakers are going to blend in with your L/R speakers (regardless of brand) and provide a seamless and convincing front soundstage. As a temporary solution, sure, but you will probably end up buying a center speaker.
Either follow JOHNnDENVER's advice and use phantom center until you can accommodate a center speaker or shop for a center from the mfg of your left & right speakers. It's important to match, as closely as possible, the timbre and sound characteristics of your speakers and the front 3 are the most important. Your Denon may have some basic room EQ adjustments in its calibration setup that can help but that can only grossly approximate what you will get with 3 "matched" speakers.
Good luck! Try both & see for yourself which is OK to your ears and go from there. My bet is you'll prefer a phantom center over the TV speakers
If u are seeking connection help to try your TV, run a cable out of your receiver's center channel output with a Y-splitter and connect the Y-splitter to the TV's audio inputs at the rear. Make sure you use the same audio inputs corresponding to the ones your receiver or player video input is using and you're set. If you want to use the TV's analog audio inputs along with HDMI video input, consult your owner's manual to see how to do that. Standard Dolby/dts surround or THX will send the center channel to your TV then.
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