Pin C.3 on a 14M2 is "Input Only", so it's strictly not an "I/O" pin and can't be used as an Output. But sorry, I don't know Blockly well enough to say if a "workaround" is needed to use it as an input.
In the Basic Compiler, an internal "Weak Pullup Resistor" can be used in some circumstances as a "Quasi (High Impedance) Output", but this isn't a feature one would expect a Blockly / beginner to use.
C.3 is an input only pin on the PICAXE-14M2 so it can be selected as an input pin but not used as an output pin.
If there is somewhere else C.3 is not available when you believe it should be; if you can let us know where that is, which block, we can take a look and investigate further.
Being input only is a silicon issue so it;'s not possible to make it an output. There are some tricks where such input pins can have their characteristics changed by applying pull-ups to allow use as outputs in some circumstances but that's not directly supported by Blockly.
I'm aware that c.3 is an input only pin like some pins on other picaxes and I will be using as an input. The i/o table is only used to assign a descriptive name to the pin and NOT set it up as input or output. It greatly enhances the readability of the blockly blocks. Important for learners (young and old like me).
This is definitely an error as other chips such as the 8m2 has all it's pins (c.0 - c.5) listed in it's table.
The 08M2, 18M2, 20M2 and 20X2 chips each also have one input only pin and for these chips all the port pins, including the input only pins, are listed in the Input/Output Table and can be assigned a meaningful name as Label.
So, not being able to be assign a label for the C.3 pin on the 14M2 chip does seem inconsistent.