Usually the "sender" for high temp and low oil
pressure alarms are really just switches that make contact if the oil pressure falls below a set level, or the water
temp goes above a set temp. This making contact or closing causes the alarm
to sound and the light to come on.
Actual senders, as used to drive gauges are variable resistances that change value as the oil pressure changes or the temp varies. This varying resistance allows more or less current
to flow thru the sender, which means more or less current
flows thru the gauge, causing the needle to go up or down. Because of this relationship between the sender and the gauge, it's necessary to have gauges and senders that are made to operate with each other, so it's necessary to purchase
senders and gauges that are matched to each other's values.
Usually oil switches and senders are made with 1/8" pipe threads (or equivalent metric sizes) so it's not terribly difficult to "T" them together or find threaded plugs in the oil gallery to add a sender to drive a gauge. Some manufacturers also make combination units which contain both an alarm
switch as well as a sender for a gauge, so that simplifies installations.
Temp switches are typically 1/2" pipe thread or perhaps 18mm thread, so it's more difficult to "T" them together. More often there are extra threaded openings in the engine
block or head
to accomidate a second sender.
I'm not super familiar with what's available on the 3GM30F engine
for extra senders, so I would suggest going online and finding an installation
drawing of the engine. Usually this will show the location of any extra ports
for optional gauges. Failing that a dealer should easily be able to point you in the right direction.