Cmd + B = **Bold**
Cmd + I = *Italic*
Cmd + U = <u>Underline</u>
Cmd + Shift + S = ~~Strikethrough~~

Alternatively you can use mark down...

Surround text with ** ** = **Bold**
Surround text with _ _ = *Italic*
Surround text with ~ ~ = ~~Strikethrough~~