The "3" is interpreted as the line color parameter. As no text-to-color follows, mIRC prints the error.
Quoting the helpfile:
/echo [color] [-cdeghiNtsaqlbfnmr] [color name] [#channel|[=]nick] <text>
Prints text in the specified window using the specified color (0 to 15).
To bypass this issue either specifiy a target (like /echo #somewhere <yourtext>) or use any switch (like /echo -q <yourtext>). "/echo
-a <text>" is the most common method. Alternatively you can prefix the desired <text> "3" with a valid color number, so the first number will be [color] and the second number the <text>.