how to use lower vdd devices at higher values

how to use say 1.2 v devices (transistors),in say 3v supply

Check the device datasheet. If the device is rated for 0-1.2V, there is no way u can run it at 3V, u need to use step down switching regulator or LDO regulator. But usually, transistors are rated from 0 to 5V at minimum. So, its ok to use it at either 1.2V or 3V.
.
hope that helps.

A device's "rating" has many facets, if you drill down into the
details. The foundry will call it out at its lowest limiting aspect.
It's quite likely that a FET has a higher Vgs and Vgd than its
long term reliable Vds at worst case HCI bias. So sometimes
(in some topologies) you can safely use a device beyond its
simple scare-the-innocent rating.

If you are using an SOI technology you can take advantage
of the infinite (for practical purposes) body-to-substrate,
hence body-body, voltage and use cascoding techniques to
divide the applied "off" voltage into bite-sized Vds portions.

The problem you run into, quickly (N>2) is that the higher
gates now require their own, and well phased, high-swing
gate drivers, and the elaborateness snowballs.

Or, you could use the "I/O" thick oxide transistors, which
might be capable as is. Or find a flow where this is true.

For small amounts of circuitry you may be better off with
an internal LDO and level shifting at the ins and outs.

when we use mos caps and when we use metal caps