When we write 3 + x, we are basically saying "3 plus something, only I don't know what the something is". The x simply means we don't know (yet) what number we're adding. So when you say
3 + x = 8
you're saying 3 plus some number I don't know about is 8. Or, in other words, "when I add some amount to 3, I get 8".
When you're asked to "find x" you're being asked to find what value it actually is that we should be adding to 3 to make that statement work. It's like your teacher or parent saying: "3 plus what makes 8?". If they asked that, you'd probably figure out pretty quickly that the answer is 5, since 3 is 5 away from 8. In math, we answer this by saying "x=5".
Sometimes, instead, you'll see another version of the same problem that simply says "What is 3+x if x=5?" Again, what they're saying here is that you should add something to 3 (3+x) and then they tell you that the something is x (x=5). So they're telling you "add 5 to 3". You know that it adds to 8, so we write
3 + x = 8 if x=5
or: "3 plus something is 8 when the something is 5"
