All addition / subtraction modifiers (the +1's) are worked out at the same time, and then any set modifiers (the maximum of 3+) are applied. If you use the improvement from Stealth Shielding to get past a 3+, you have broken a rule.
Note that this is the same as the Mark of Tzeentch (which improves invulnerable saves by +1 to a maximum of 3+), and a 10-page thread on the topic
(from 2013, but nothing really has changed in the relevant rules) with a 51%/49% split poll can be found
here. So bear in mind that the community is split on it.