When Antigonus started disbanding the silver shields, he sent them by small groups of two or three on dangerous missions in order to rapidly dwindle their numbers. What missions were the silver shields sent out to do?

Diodorus doesn't mention small groups, but says that Antigonus chose 1,000 of the unruliest Silver Shields, who were assigned to the satrap/governor Sibyrtios for the frontier war in Arachosia. They were to be treated as 'expendable', so he must have hoped this would get rid a lot of them.

The sources for this are Diodorus 19. 48.3-4, Polyaenus 4.6.15, and Plutarch, Eumenes, 19.2. Polyaenus says that the rest of the force were assigned to garrison duty in various 'out of the way' forts.

There is a bit of information about this in 'Antigonos the One-Eyed and the Creation of the Hellenistic State' by R. A. Billows

Edit: sorry, I've just checked another translation and it does actually say 'he [Antigonus] privately directed the satrap/governor to send a few of them at a time on duties in which they were bound to be killed', but nothing hints at what those exact duties were