3891 yes, silverstone or other non-Dell SFX or TFX 24/4 pin psu plus 24 to 6 pin adapter  3910 not currently because the Dell proprietary 8 pin for motherboard has no adapter.  Dell makes a 750w psu that has 10+4+4 pin.  not certain it would work for the 8+4 pin of 3910 motherboard 
Is there a power supply of at least 750 watts available for the Inspiron 3891 and 3910 desktops? It does not matter whether the psu is from Dell or another source. A plug-in Dell or a commercial small form factor psu with an adapter cable would be fine. It seems incredible that these nice computers are hobbled with (at most) 300 -watt power supplies.  Thank you.
Power Supply for Inspiron 3891 and Inspiron 3910