sorting/comparing column values in non-alphanumeric sorting ways ?

Hi:

I have a table listing tools and tool versions for a number of different
tool configurations. Something like this...

create table tv (tool text, tcfg1 test, tcfg2 text, tcfg3 text, highestver
text);

insert into tv (tool,tcfg1mtcfg2,tcfg3) values
('tool_a','1.0.5b','1.0.10','1.0.9');

I want to load the 'highestver' column with the highest version of tcfg1-3.

This won't work...
update tv set greatest = greatest(tcfg1,tcfg2,tcfg3)
...because it thinks 1.0.9 is greater than 1.0.10

Is there a way to get this to work right ?

Thanks in Advance !