text substitution in a colum

Hi,
 
This is my firts port.
 
I have a file with colum like this.
 
Title1    Title2      Title3       Title4
AA          B            66             D
BB          DS          7              TT
VS          FDSE     53             TGF
BG          F            5               GF 
 
I would like to have with a sed command the following result. Add the
character MY_ before all numbers in the colum number 3 and preserve the
title "Title3". All the colums are seaprate by a tab.
 
Title1    Title2    Title3    Title4
AA         B         MY_66      D
BB         DS       MY_7       TT
VS         FDSE   MY_53      TGF
BG         F          MY_5       GF 
 
 
 
Many thanks for your help/example.
 
Regards.
 
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