R, gsub(), replace "." for "," given some conditions

I have data like

obs-----Var1   
1-----36.091  
2-----14.678  
3------3.12

My problem is that i want to replace "." by a "," with gsub(), but not for all the observations, just for the observations that have 3 characters to the right of the dot ("."). In this case for observations 1 and 2.

I'm thinking of something like:

if ( nchar(something indicating the characters to the right of ".") >= 3){
gsub(".",",",data[,1])
} else {}

I don't know if it's the correct approach, but i don't figure out another way.

Thanks