RF Throw Port 3. This pin is dc-coupled to 0 V and ac matched to 50 Ω. No dc blocking capacitor is required when the RF line potential is equal to 0 V dc.
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I assume you are referring to an RF signal of some frequency with a peak positive value of zero volts and a peak negative value of -0.350V, correct? As they are DC coupled to 0V, a non-zero DC signal on any of the ports would short to ground (without a blocking capacitor), which would likely damage the device, and placing a blocking capacitor on a DC signal would be pointless.
I would say that your scenario is undefined within the datasheet, because your signal’s average value is a bit negative, though not highly negative. Nevertheless, since its average value is not zero volts, it would seem prudent to add blocking capacitors to keep the part operating within characterized parameters.