You could calculate the distance between the tensors using torch.norm and use argmin to get the index corresponding to the smallest distance:
a = torch.stack([torch.tensor([1,1,1]), torch.tensor([5,5,5]), torch.tensor([10,10,10])]).float()
b = torch.tensor([2,2,2]).float()
min_idx = torch.norm(a - b.unsqueeze(0), dim=1).argmin()
print(a[min_idx])
> tensor([1., 1., 1.])