I am trying to figure out once and for all how the GPLv3 interacts with web applications, specifically SaaS applications. I haven't been able to find a concrete answer about this.

Let's say I have a SaaS web app that users can purchase a monthly subscription to and access. The source code is not distributed or made available to the customers in any way, shape or form and their only interaction is via the website and any APIs that I've provided via web services.

If I am using a third-party library licensed under the GPLv3, let's say a UI toolkit like Telerik's "KendoUI" (the open source version of which is licensed under GPLv3), then am I required to provide the source code for my web app (and therefore, in effect, make it worthless to sell as the source code would be readily available)?

I am aware that the Affero GPL does require this and was specifically created to close the "loophole" wherein you could use open-source libraries in a web app and never have to release your own source code (which IMO kind of defeats the purpose of a web app, but neither here nor there), but it doesn't appear that the standard GPLv3 license has this clause (in fact it appears to have been taken out specifically because of the backlash it would cause).

Is this understanding correct? In other words, unless I want to open source my proprietary application, I should steer clear of anything using the Affero GPL, but the GPLv3 and others are fine as the "loophole" still exists within those?