Can I use a 27.5 fork on a original 29er (2015 Cannondale Tramount 3)? I have a Cannondale Tramount 3 e-bike that I would like to make better and lighter in every way I can for as little cost as possible, but with quality used or discounted parts. 

Can someone help me with a tip for which fork to use (now it has a Suntour XCR, which is over 2.6kg) I think that with a 27.5 it will stand a litle more on its head but I think I still will have place for a 2.3 x 29 inch wheel. Originally it had 1.9, now it has lightweight 2.2 Maxxis tyres.
The key to swapping forks is the axle to crown measurement - the distance from the center of the front axle to the seat of the crown race that is the lowest part of the headset. You want the replacement to be as close to the original as possible, to preserve the designed geometry. There are forks that can take either a 27.5 or a 29er wheel.
The suspension fork is heavy, if your goal is light weight then you should consider the correct length carbon rigid fork. You will need to balance weight, cost...and what is available. Remember that you need to find a fork with the same steerer diameter - you can't put a tapered fork on a bike built to accept a standard 1 1/8" steerer.
The difference between a 27.5 and a 29 can be large or small, depending on the tire size you choose - if you put a 27.5 wheel on the front with a 27.5 plus tire (3 inches wide) then the out diameter is nearly the same as a regular 29er tire, with a rigid fork you would have some suspension, too, without the weight.
Good luck!