Welcome to Switzerland, even if it’s for very short time 🙂
There is no need to travel to Zurich if you want to go to Paris. Take the direct train Interlaken West to Basel SBB and change trains there. The TGV is servicing Zurich-Basel SBB-Paris Gare de Lyon. Travelling via Zurich would mean a detour.
Since you are staying 3 days, I suggest to buy the 3-days Swiss Travel Pass. It covers all travel in Switzerland, including Grindelwald. If you want to visit Jungfraujoch, it is granting a rebate for the Grindelwald-Jungfraujoch stretch. With the Swiss Travel Pass, there wil be the complimentory Swiss Family Card. I suggest to buy the tickets upon arrival in Geneva. The train station and the SBB travel center are located right in the airport’s building.
Instead of going to Grindelwald, you could visit Mürren. It offers great views on a sunny day. And it includes a cableway (included in the STP). Interlaken-West to Lauterbrunnen, then cableway up to Grütschalp. From there, small train to Mürren. You could stop at the intermediate station “Winteregg”, for photos and lunch.