1 x 3 splitter with connecting cables for Wi-Fi RF calibration
There will be relections from any impedance discontinuities, how severe depends on how precise the connector impedance's match the cables, amplifiers and splitters.

You might find getting some circulators useful or at least adding some pads between stages to improve the isolation.

The VNA is the right tool for this kind of thing, as it can usually read out group delay directly in ns as well as measuring S11,S22 directly which is useful for checking SWR. 

I hope the sample clock to your ADCs is delay equalised?