I agree with most of the above, especially the comment about how usefull VFD output current is taken alone. But I want to point out a simpler solution to reading output current (if you must). Use an old-fashioned analog clamp-on meter, like an Amprobe. For the size of motors you mentioned, they will work fine.
That said, did you understand the points made about the non-existant relationship between input and output in a VFD? This is probably the most commonly misunderstood concept. The VFD uses input power simply as a "raw material" for it to produce an output that will make the motor respond in the desired way. Think of a VFD as an entirely new power source, one that just happens to get it's supply from the same utility as the rest of your plant. Whatever is happening on the line side of the VFD, i.e. voltage imbalance, spikes, phase reversal etc., has no effect on the motor connected to the load side. It only affects the VFD itself (with one exception as follows). In fact with regards to "imbalances", other than an increase in input current most VFDs will work fine with only 2 of the 3 phases feeding it! The one exception is that a VFD cannot create a voltage potential beyond that of the input, so if you have 440V voltage on the input, you cannot have 460V on the output.
"Venditori de oleum-vipera non vigere excordis populi"