How to feed training set labels into Keras LSTM

Your approch so far is not exactly right. Assuming you want to use the many to one approcah, the way you do it is as follows:

You have a dataset like this:

And you prepare your data in a way like this:

Source

For a timestep of 3, you discard the first 2 occurrences of your labels and use $y_{3}$ for the inputs ${x_1, x_2,x_3}$.

Next you don't start in $x_4$. Instead you use the next three-item slice ${x_2, x_3, x_4}$ with the training label $y_4$.

You continue this way until the last imput ${x_{n-2}, x_{n-1}, x_{n}}$ to predict $y_n$. If you have $n=30000$ samples, you can generate up to $n-2$ (29998) training samples for your model following this logic.