It is basically an offer where you will get 90 days of unlimited data, calls and SMS. You won't have to pay the initial cost to avail this offer, but you will be getting this offer, only if you buy it from Flipkart and the SIM and the connection you can avail it form the reliance digital store.
LYF Wind 3 is not that bad, just its performance is not that fast, and its battery support is also average. But go for the LYF phones as they are a perfect match to reliance Jio offer. I have seen that most of the phones lack the reliance jio SIM and the ones that support the SIM are quiet expensive.
As far the offer is concerned, the Reliance JIo offer gives you free data, voice call and SMS for the first three months. And the packages and offers are quiet budget friendly.