Why can I not get an accurate transfer function?

I am in the process of trying to gain a transfer function for three known variables of a system (pressure, temperature & flow velocity) and relate them to the rpm of the low pressure rotor of the system (system is a turbofan engine).

I have attached images below of the data vs rpm... (in the order of T, P then V).

For some reason, I can not get the system identification toolbox to create a relationship between these variables in the form of a transfer function, even though it seems to me it should be as simple as a gain? (I am new to using matlab and the toolbox itself so I may be completely wrong...)

Does anybody have any ideas as to why this is the case? (the accuracy of my transfer functions tend to either be negative(???) or small (20% or less)).

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Eventually, I would like to use a transfer function matrix to gain a single best value of rpm (output) from the three known values.

Thanks in advance for any help...