No, it's nonsense. That is completely untrue and people get boys after 3 girls all the time. For some reason that escapes me I think medical professionals are largely opposed to bigger families and are willing to let their opinions affect the type of advice they give.
Please check out this link, your odds actually go up after 3 girls!!
What are the odds of having another boy or girl? : Ingender.com
I know people who had boys after 3,4,5,6,7 and yes, even 8 girls (and she had 2 boys in a row!) My own great-grandma had 6 girls and then 2 boys at the end. It's just not true.