Hello,
I am US Citizen and I will be travelling to England for 3 days and then visit Switzerland for three weeks , then go back to England for another 3 day stay before taking flight back to USA.
I understand I will not need VISA to travel to England but is it a problem if I enter England again within 3 weeks?
In a word, NO.
As a US citizen, you are allowed 6 months in the UK (England is part of the UK). You are allowed 90 days in any 180 in the Schengen Treaty Area, of which Switzerland is part, the UK is not part of Schengen and keeps its own rules for entry of foreigners).
The same rules apply whether you are flying between Switzerland and the UK, or travelling overland (train, or by road).