The emoticon <3 typically stands for a heart, it represents love and affection, you have to tilt your head sideways to see it. Link
I used to think it looked like a cone with two scoops of icecream. Your question is probably off topic for this site, but you said you researched, and I can see why typing <3 in Google might talk about numbers being less-than 3
The less-than sign is a sign of inequality. The less-than symbol is used in various operations that usually pertain to work being done mathematically or with a programming language. The symbol looks similar to a sideways "V" and is generally used to show inequality between two numbers or expressions.
It seems very odd that the folks at SE should write use that emoticon to close their email, maybe they wanted to sound friendly.