OK, pet peeve here folks:
For this game,
Linear growth is generally achieved by flat bonuses. +1 bonus increase that stat by 1, not matter what the value of the stat is. This can also be called incremental growth.
Polynomial (or multiplicative) grow is what % bonuses usually do: they increase that stat by a certain amount. Each +X% increases the stat by the same amount as the last +X%. That is, it's Stat * (1 + (X + Y + Z)/100), where X, Y, and Z are % bonuses.
Exponential (or geometric) growth doesn't exist in this game. We would have each bonus either increase the base stat as an exponent function - e.g. (stat)^(bonus) - or it would be % bonuses that operated in sequence, rather than additive - e.g. Stat * (1+ X/100) * (1+Y/100) * (1+Z/100). To my knowledge, NOTHING in the game behaves like this.
SubExponential (or Bounded) growth would be using something like a log function or an exponent with value < 1. Right now, I don't think GC3 uses this, but it used to be how Population was converted into Raw Production (e.g. using Square Root).