Is it possible to connect to a Pi3, over Bluetooth for SSH, for iOS? I know you can connect to it with BlueZ, and SSH is possible over wifi, but I can't find anything about doing both. The advantage would be to do it in an area without wifi. Maybe it is possible, but there is not an app for it on iOS?
No, it is impossible to set up SSH over Bluetooth. Even more so with Apple's Bluetooth implementation on iOS.
SSH works over a regular TCP/IP stack implemented on Ethernet or WiFi family of protocols. Bluetooth implements its own protocols with different purpose.
You can however configure either the Raspberry Pi or the iOS device to serve as a hotspot and connect to it with the other device(s).
for Raspberry Pi you need to install `hostapd and configure it to enable WiFi network. There's a good guide Using your new Raspberry Pi 3 as a WiFi access point with hostapd
for iOS go to Settings -> Cellular -> Personal Hotspot, turn it on, set the password
After you connected two devices you can use regular network tools (including SSH) in your ad-hoc network. You could then use an SSH client application on iOS.
With a hotspot on an iOS device you need just to take care not to (over)use your data plan for Raspberry Pi communications (like updates).
I don't know about iOS but you definitively can with do SSH over BLuetooth on a laptop. You just need to setup a PAN (Personal Area Network) which is emulating an Ethernet-Connection. There you get a virtual NIC (Network Interface Card) and then you can SSH into your Raspberry Pi.
See Bluetooth Network on eLinux. I'm not sure if this is the most up to date version. I can provide more links if you want to give it a try.
Bluetooth PAN Network Setup with BlueZ 5.X (Use Pi as host)
Raspberry Pi 3 Bluetooth Networking (Using macOS as gateway to the internet)