How to sign my own public key?

[Stayvoid]

I'm using GPGTools (Mac OS version).

I've tried to verify the public key with Services - Verify (GUI method).
(I assume it's used for verifying a signed message.)

Just to clarify, here is an output of the gpg --list-sigs:
pub ***
uid ***
sig 3 ***
>
sub ***
sig ***

Do I have any unsigned keys?
"sub" stands for the secret key, right?

Are there any differences between sig 3 and sig? (Those keys have the
same output.)

Can I accidentally encrypt my mail using my own secret key? (I assume
it will become unsafe after that, right?)
For example: gpg -se
What kind of key will be used in this case?
I know that the program will ask for the User ID. Will it
automatically use User ID's public key?

Thanks for your help.