DiGi broadband and 3G are 2 different services, most people are confused. This is the information that I have collected these 2 days, hopefully can help you to have a better understand the 2 different services.
DiGi has no 3G services, the DiGi 3G is just a marketing term. The DiGi 3G is a 3.5G (HSPA, High Speed Packet Access or some name it HSDPA, D means download), which is faster, an improvement to the 3G services. In general you can call 3.5G and 3G as 3G services, but DiGi 3.5G services only provide data access. You can not have conference call with DiGi 3G (3.5G) services. If you have a phone which only support 3G, it will not work with DiGi network.
To help the consumer have a better understanding on DiGi broadband and 3G, DiGi people will tell you that broadband is for your desktop access to the Internet, 3G is for your phone to access to the Internet. If you just normal people and doesn't want to know more, you should stop here.
Further question, can I use my desktop or notebook connect to the DiGi 3G services? The answer is yes. You can place your SIM card (with 3G service activated) in to GSM modem which support 3.G (eg. Huawei 160e), or you can use your mobile phone as a dial-up modem for your notebook, it works. The only thing missing is you will not be able to receive phone call while accessing the 3G services with your notebook.
How about the other way? Will it works if I place the broadband SIM card into the phone? According to the DiGi representative, it works as long as your phone support 3.5G services, but you can not make calls with it. Since I do not have a mobile phone which support 3.5G, I have not tested with it.
If you access to DiGi broadband web page, you will notice there are 2 data services call broadband and 3G, hope my explanation can help you to make your choice.