  We prepare a triple quantum dot with a separate contact lead to each dot to
study Pauli spin blockade in the tunnel-coupled three dots in a row. We measure
the tunneling current flowing between the center dot and either the left or
right dot with the left and right leads as a common source and the center lead
as a drain. In the biased stability diagram, we establish Pauli spin blockade
in the respective neighboring dots, with features similarly obtained in double
quantum dot systems. We further realize Pauli spin blockade with two different
conditions by tuning the inter-dot coupling gates: strong and weak inter-dot
tunnel coupling regimes. In the strong-coupling regime we observe significant
suppression of co-tunneling through the respective double dots due to Pauli
spin blockade. We reveal the influence from the third dot in the triple dot
device on this co-tunneling Pauli spin blockade and clarify that the
co-tunneling Pauli spin blockade is lifted by the resonant coupling of excited
states to the third dot level as well as spin exchange of the left and right
dots with the adjacent reservoir.
