What the code is doing as the manual shows, it is triggering of channel 10, but measuring channels 1 & 3, so to answer your question in full:
1. Triggering off channel 10 to start the measurements
2." ...If take reading from 1 and 3 why only call channels(0) only?"
This is the address of an array, listing the channels to
be used (so it has channel 1 and 3 included in it)
3. You do not have to explicitly declare the variable, the first time it is called it is declared automatically.
The values are stored in an interleaved fashion, so location i is for channel 1 and location i+1 is for channel 3.
4. If you wanted to measure just channel 5 you would need to make the following amendments:
change channels(0) to = 5 rather than 1
remove channels(1) = 3
Change the last parameter to 1 rather then (since you are only measuring one channel)
Call adc11_set_interval(10000, 100, channels(0), 1)
In the for loop remove the line below , since you are only measuring one channel.
Cells(i + 4, "C").value = adc_to_mv(values(2 * i + 1))
Hope this helps.