After you encountered 1 and 2 from Number Lore, they both decide to use the Math Machine from the calculator to create a new number, 1 goes in the first glass container, the sign between the first 2 glass containers changes into a +, then 2 goes in the second glass container, and when they were done, 3 - Number Lore appears closing his eyes, but when he opens them, his mouth looks like he is about to burp like a frog

Ree-thr

And then, he speaks up his name after the burp

Threeeeeeeeeeeeeee!!