What is wrong with this code line “$(‘#myid\.3’).text(‘blah blah!!!’);”

The problem with above statement is that the selectors is having meta characters and to use any of the meta-characters ( such as !”#$%&'()*+,./:;<=>?@[\]^`{|}~ ) as a literal part of a name, it must be escaped with with two backslashes: \\. For example, an element with id=”foo.bar”, can use the selector $(“#foo\\.bar”).
So the correct syntax is:

$('#myid\\\\.3').text('blah blah!!!');