How does your phone connect to 3G if that has been turned off? If there is no 3G signal it should connect to 2G (if your network is still broadcasting 3G but not offering a service then they are morons!).
Anyway, I don't think you can, or at least it's at your own risk to try. I've looked in the hidden "testing" menu (type *#*#4636#*#* in the dialer and select "phone information") and the only option I can see in the "set preferred network type" that includes 4G (LTE) and 2G (GSM) but not 3G (WCDMA) includes protocols that are only available in China and which I'm not aware that my phone's hardware supports, so I'd be wary of selecting that option in case it also had some other side-effect. If you do decide to try this please make sure you note what the current setting is first, and do not change anything else: this is an engineering menu, not something that is supposed to be touched by the general user, and if you mess up the settings in there you can lose basic telephony features. It's one of the few places on an unrooted phone where it's possible to really mess things up in a way that isn't easy to fix, and I've seen people who didn't take sufficient care with this menu do just that. But it's also the only place in the phone where you can make changes to the network that go beyond what's available in the regular system settings (which on my Pixel offers 4G or 3G, nothing else).