At Schiphol, is 3 hours enough to get from EasyJet to Delta/KLM for a return flight to America?

Three hours is enough time to make basically any connection at any airport, particularly one which doesn't involve immigration checks. You will have to go through exit checks, but that's pretty quick, and Schiphol isn't gigantic. Assuming your luggage is checked through to the US, you'll likely have more than two hours of excess time. Even if you had to exit to landside, re-check your baggage, and go back through security, you'd still make it comfortably.

Of course, if your incoming flight is delayed by a huge amount things could change, but that's true of literally any connection.

Arriving from France on EasyJet you need to get out of the passengers only part of the airport, that is through the area where the luggage is returned (and do not forget your luggage), through the customs area (where the door with 'nothing to declare' are) and out into the shopping area around the arrivals exits.
From there you need to go up one floor and find your right entrance.

If possible check-in online so you can skip the wait for the counters. Behind the area with the check-in counters is the area where you can hand in your luggage, the most used by KLM is a system where you need to do the work yourself.
From there you will have to go though security and exit passport control both of which can have plenty of people waiting for them.
Then it is a short or long walk and as you are going to the USA, likely an extra security check just before getting on the plane.

If it is not busy, your 3 hours will be enough, just under one to get out, two to get in and to your gate in time for your boarding call.

On the other hand, if you do not have hold luggage and are checked in for your flight, you might be able to stay air-side and the whole will be easier.
(Warning, according to someone who has used that part of the airport, this will not be easy and you should ask for assistance if it is possible at all.)
In that case you will not need to go out of the airport, but will need to get to connections and from there to your gate. Somewhere on the way you will pass through exit passport control and the only security you will see will be at the gate.
As Schiphol is my nearest airport I have never gone from one part of the airport to the other and I would not know whether KLM would insist on you arriving from outside. (As far I can see they do not care if you have your boarding pass, but I am not 100% sure.)

With an EasyJet flight you will not have any help if you arrive late, it is your responsibility to have enough time between flights and I think that 3 hours is asking for problems. Delays are so common and even flights leaving earlier than advertised when you bought the tickets (for your second flight) I would try to add your France-Schiphol ticket to your Schiphol-USA flight, so both are by KLM and partners, where the company will take the risk of you missing your flight.