It depends on the exact use case or requirement and type of device in hand.
In general, the device will choose/use the wireless connection with better connectivity strength as per its design/specification and also it depends on the support at device.
In case you have the option of WiFi Hotspot tethering enabled, it is possible that most of the devices nowadays allow you to use 3G for the internet communication and at the same time, it allows WiFi hotspot sharing with the help of WiFi from your device.
Also, few devices have support for file sharing between device to device whereby they provide support for wifi direct and wifi sharing while 3G connectivity with internet is still fine. Few devices from Samsung, support an application called "Download Booster" that allows your device to download files using WiFi connection and a mobile data connection simultaneously.
Few devices from Apple starting iOS 7 introduced Multipath TCP enables users to have their streaming music to never get interrupted while on high mobility or in trains and other vehicles with changing connectivity whereby no manual switch from between WiFi and Cellular is required in which case, the device shall stay connected over both cellular and WiFi simultaneously and whenever the WiFi connection fails, the cellular connection shall continue with streaming operation without any interruption.
There is also a app called 'Speedify' in android that uses channel bonding technology to combine multiple Internet connections together for increased throughput & redundancy and thereby it can merge WiFi and Cellular connection simultaneously. Here, a Speedify client software is used to establish a connection to a Speed Server in cloud that acts as a middleman between you and the rest of the internet. The Speedify client software then works together with Speed Server to distribute your internet traffic and deliver the combined speed of all available internet connections.