Voltage from batteries in series operating on 3 ATTiny, 3 Buttons, and 3 Piezo buzzers I intend to power three ATTiny45 chips (each requiring around 3.5V) in parallel with 2 pill batteries (namely Maxwell's CR2032) in series (which would come to around 6V). They will each play a sound with a piezo buzzer when a button is pressed.
Now maybe my thinking is wrong but I think dividing the power between the three chips may result in only around 2V delivered to each chip? Does the other components play a role?
So my question is this:

How much voltage is delivered across elements in parallel?
What changes should I do to the wiring to make the most of the voltage?
What kind of resources should I get to make sense of what to do? Cause I thought I was more advanced but this stomped me.

In any case I've added the schematic of what's intended.
[This schematic used to contain resistors to pull the switch but this need was removed with the help of jsotola's input_pullup comment]



simulate this circuit &ndash; Schematic created using CircuitLab
If you connect the AtTiny chips in parallel, they will all receive the full voltage from your battery. If you exceed the current output of your battery the voltage will droop.
If the AtTiny chips need 3.5V and you give them 6V instead, it will likely destroy the chips.
The short answer: Don't try to drive microcontrollers directly from batteries. You should get a 3.5V switching power supply. Then you will be able to drive all 3 chips cleanly from your battery. (Switching power supplies are very efficient. They don't waste much power at all.)
Note that button cells produce very small total power (in milliwatt/hours) so they won't drive your processors for very long before they are exhausted. I suggest using 4 AA batteries and a switching regulator instead. The 4 AA batteries would give you a 6V VIN to your voltage regulator, but a lot more total power.