Odin3 Bootloader button is greyed out
In the case you weren't eager to look at the research, here's the solution.

In the same folder of your Odin executable, you'll find a file called Odin3.ini. Being it a configuration file, it defines the behaviour Odin should meet at startup. What we're interested into, though, is to examine its content, so fire up your text editor of choice and open the file.

Once opened, you'll see various sections, each marked by a [Something] label. What you need to do, is to find the [ButtonOption] label.

After you've found it, you'll see various entries under it. Specifically for your case, the section should be similar to the following:

[ButtonOption]
Bootloader=0
PDA=1
Phone=0
CSC=0
UMS/PATCH=0
USERDATA=0


Now, a 0 means that the corresponding checkbox is greyed out, while a 1 lets Odin know that such checkbox will be selectable. Since the flags' names can be confusing, here's a brief overview of their meaning:


Bootloader = BL button
PDA = AP button
Phone = CP button
CSC = CSC button
UMS/PATCH = UMS button
USERDATA = not implemented yet


You should now be able to edit the flags in order to activate the needed checkboxes. Just remember to save your changes afterwards.