So you want BOLD [b], UNDERLINE [u] and ITALIAC [i]?

[Greg]

Using the content replacement in settings panel, Look & Feel>>Add Replacement.

BOLD TEXT (and more) USING CONTENT REPLACEMENT:
Source Content: [b]
Content Replacement: <b>
Replacement Type: Text
Source Matching Method: Match any location in message.

If you want to also use the end tag for bold create another content replacement similar using:
Source Content: [/b]
Content Replacement: </b>

For italic [i]
For underline [u]

All three font styles will use all four settings listed for BOLD