Are you going to visit those cities in the aforementioned order? I would start in Amsterdam, go to Paris and then to Switzerland. Or did you book flights already? Where do you land/will leave from? 

The Eurail pass works with travel days. On each travel day, you can take as much trains as you want. Usually the more travel days you have, the cheaper it is per day.

You have flexi passes (use x travel days during month/2 months/ 3 months) and continuous passes (15 days/month/ 2 months/ 3 months of consecutive travel days) 

Then I only count two big travel days:  Rotterdam - Paris and Paris - Switzerland.

Paris-Versailles not included in the pass, but isn't expensive anyway. Price of a short RER ride. 

Rotterdam - Amsterdam might be bought as separate tickets, but depends on the total amount of days you'll need. 

Two options: the direct high speed trains with high reservation cost, or slight detours or extra changes to minimise reservation cost (Paris-Lille via TGV and then -Antwerpen-Amsterdam by ordinary IC trains, without reservations cost) and take a TGV from Paris to Mulhouse or Strasbourg, and continue with local trains to save yourself a horribly expensive reservation cost across the Franco-Swiss border

Switzerland is quite expensive so a pass is usually good value there.

By the look of it: I suspect or 7 days pass will suit you best, definitely no need for a 15 days continuous pass. Although my advice can change if you answer my questions on where you need tonget from/go to to enter and exit Europe :) 

One thing to consider: When buying regular tickets, you need to buy these very early (as soon as possible) to get a good price. And you loose flexibility. 

As you did ask for the best economical option, here are some examples for 2nd class travelling:

For Paris - Amsterdam (direct trains) you may get tckets for 35 € 2nd class if bought right now. With a railpass you need a supplement for 32 € 2nd class anyway.

For Amsterdam - Zürich you may get tickets for 49,90 € 2nd class (very limited availability and higher price for most connections). With a railpass it’s possible to go without reservation / supplement. 

For Paris - Versailles you’d need a ticket anyway (3,20 € one way per person).

Amsterdam - Rotterdam is 17,90 € (regular price one way per person). When going off-peak and booking at least 4 days before, you may get this ticket up to 20% cheaper. There are day tickets, too - e.g. a day ticket for train, bus, tram and metro throughout the Netherlands for a whole day for 48 € (not valid weekdays morning) or 70 € (valid all day).

Regarding Switzerland, normal tickets are very expensive. A day ticket for Switzerland is 52 CHF 2nd class per person per day (for train, bus, tram and most ships) if bought right now (gets more expensive when waiting).

Count your travelling days. If you’d like to do longer journeys when staying in the Netherlands and Switzerland, a railpass is the better option. It also depends on age (a youth pass is cheaper).

Hi, yes we are visiting all these places - and mostly in this order as the flight from home country is cheaper if we start from paris.

Additionally, I understand that global pass may not be required as local commute is easily available and intercountry commute can be booked separately from point A to Point B as suggested. Now the only question is whether within Switzerland the intracountry commute would require swiss country travel pass and how much will it cost?

Also to answer - we are staying 4 days is paris, 3 days in Netherlands and approx. 6 days in switzerland. in this situation what would be feasible

A) global pass - or 
b) swiss country pass + separate tickets from point a to point b
 

thanks in anticipation!

If you'd like to travel in Switzerland on six days anyway it doesn't make any sense to split.

As written before: A day ticket for Switzerland is available for 52 CHF (x 6) if bought right now (a swiss travel pass youth is a little bit cheaper for six days, a swiss travel pass for adults is a little bit more expensive).

For a rail pass, as written before, the price depends on your age. For example: A 2nd class global 7 day youth flexi pass is 264 €, for adults 352 € (if bought this year). If you qualify for a youth pass, this is the best deal. 

Thank you Hektor and Brendon!