3D WebView works by embedding a browser engine, and each browser engine it uses (e.g. Chromium, Safari, Gecko) has the same security features that keep a normal web browser safe:
3D WebView preserves the full security properties of the underlying browser engine. The browser sandbox cannot be disabled, and the web security restrictions are enabled by default and are only disabled if the application explicitly disables them. So, the result is that 3D WebView is as secure as using a normal browser, like Google Chrome or Safari.