From the above information, I think your input layer would be a featureInputLayer. So according to your training data, the output of read operation on the combined datastore should be as follows:
>> read(cds)
ans =
1×2 cell array
{3×1 double} {5×1 double}
For more information you can refer to the documentation of trainNetwork and desciption of training data format for feature data in case of a datastore: features - trainNetwork.
I am attaching code to generate random training data:
f = 3; r = 5; n = 5;
layers = [featureInputLayer(f)
fullyConnectedLayer(r)
regressionLayer];
xtrain = randn(f,n);
ytrain = randn(r,n);
xds = arrayDatastore(xtrain,IterationDimension=2);
yds = arrayDatastore(ytrain,IterationDimension=2);
cds = combine(xds,yds)
