Reading two sensors at different frequencies... multiple Timer3?
The rule is:


One Timer, One Interrupt.


When the timer overflows it triggers its interrupt. It only has one. That interrupt then runs your ISR. 

If you are using a library to provide abstraction from the timer registers then it is down to that library how it calls your ISR. It is possible for one to provide a mechanism whereby you can "attach" multiple routines to the same interrupt, but that is quite a lot of work and overhead for a simple timer library to provide, so it is highly unlikely anyone would ever bother.

So how do you do multiple sensor measurements from one timer? Simple: just do multiple sensor measurements. You don't have to restrict yourself to just reading one sensor within your interrupt - you can read as many as you like. There is just a couple of caveats though:


The whole ISR must take less that 10ms to run or it will overrun the timer overflow period.
You cannot do serial from inside an interrupt*


So in your interrupt you read the sensors and add their values to a buffer. Then in your main loop you read that buffer to send the data out through serial. You may want to set a flag in your interrupt routine to alert the main loop that there is data available, or implement a circular buffer which has an equivalent of a .available() function to see if there is any data to read from it.



*You can place data in the serial TX buffer but it will not be transmitted out of the TX buffer until after the interrupt routine has finished. If you put too much data in that TX buffer it will block indefinitely waiting for it to empty.