1 circuit 2 switches?
You can branch off the 3-way switched circuit of the can lights to the a new single pole switch that then in turn switches the reading light can. However in this scenario the reading light can light will always be OFF when the rest of the can lights are OFF. 

There will not be a straightforward way to mix the two 3-ways with a 1-way to control the added can light so that the the added one can be ON when all the other can lights are OFF. The simplest is probably going to be to tap into a live circuit power source and then just independently control the reading light with that one way switch. 

There are some types of smart switch setups that can be configured to get the can lights to work somewhat together and still allow independent control of the reading light but you would have to carefully research the various technologies and find the one suitable for your application. Be prepared to spend a fair amount of cash as the typical smart switches cost upwards from 8 to 10 times what a common manual switch would cost.