A first task to address would be to clean up the code from all those useless Strings
String tempConv1 = String(v1int); // construct string representation
tempConv1.toCharArray(v1char, 5); // string representation is 4 chars plus null terminator, copy 4 characters, ignore null terminator
You are shooting holes all over the place in the Heap and if I remember correctly I don't even think that toCharArray adds the null at the end of the charArray for you, so v1Char is not properly terminated. I don't know if that null termination is important or not in that code, but from a memory management standpoint what they do is poor.
What the code segment above does is just put in v1Char the ASCII representation of v1int that you know is max 4 digits. So this is a great place to use itoa() which Converts an integer value to a null-terminated string. So you do just itoa(v1int, v1char,10);
At the very end, if I change tempToRead from 0 to 3 so it correctly cycles thru all 3 MAX31865 chips, it screws up operating
Just thinking aloud here... I'm on my phone so can't see the full code - thus this comment may be totally irrelevant - but if 0 means read one MAX31865, shouldn't the max value you put in that variable be 2, to read 0,1,2 which are 3 values? And if you put 3, won't that be reading 0,1,2,3 with 3 not existing thus leading to unknown results or waiting for timeout for something inexistant not answering?