My 3000 watt inverter says "lup" on the display

That means that at charge the batteries at displaying12.4 volts, but there is no power behind them, so when you apply a load, the voltage will quickly drop, even faster than the alternator can compensate when the engine is running. The inverter will detect the drop in voltage and throw a code to protect the batteries.

Unit is shutdown with display showing "OPP"

Inverter is short circuit and has shut down.

Reset to normal operation by using manual switch

Assuming you have a 12VDC In 120V AC out 3000W inverter.
LUP indicates battery voltage is low, so firstly make sure:

1. You 12V battery is good, it's large enough a minimum of 75AH and probably much larger is recommended and it's fully charged.
2. Verify you are using the correct size copper wires from battery to the Inverter DC INPUT(s). If you keep wire short the wire size can be smaller, but if the DC input wires are longer then you will need to use thicker copper wires.

At MAx load you would be taking 250 AMP from battery, so for a 5% Volatge drop of .6 V for a 10 ft length of cables from battery to Inverter you would need to use a 1AWG copper cable.

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.4066&voltage=12&phase=dc&noofconductor=1&distance=10&distanceunit=feet&eres=125&x=37&y=8

OLP is a overload condition, which means you may have to many high powered AC powered things connected to the Inverters AC outlets or maybe something that has large start up surge, ie devices large electric motors.

Or it could be all due to a faulty Inverter.

When readout shows “LUP”, it is indicating a fault happening to the unit. This protects the battery from being over-discharged. Turn off any devices that the inverter is powering. When input voltage rises to 11.7 - 12.3 V DC, inverter restores to normal.

this is a question for a qualified electrician
or ring the store where you bought it from

you need at least a 1900 watt inverter with a fully charged battery to run a 900 watt microwave,it uses 900 watts to make microwaves and it uses 700 or so to run everything else.

Could it be L.U.P. it's really L.V.P. but the V looks like a U. If it is it's low voltage protection. If the incoming voltage drops below 10.5 volts, it begins to flash. If the voltage drops below 10 volts it shouts down. If you supply it with 12 v it should restart. If you don't have manual you can view/download one from www.cobra.com.
The low voltage could caused by low battery. Poor connetions. Wire too small for load being supplied.Inverters over 1500 watts will require #1/0 or larger cable, in the shortest possible length. NOTE: Cable size recommendations may vary among inverter brands and models; check the Owner's Manual for the model you purchase before you buy the wire for it.

a very thick one,,,like a baby battery lead on your car,,,its a very bad idear to have it so far away from the battery point, you would be far better off finding a closer place to put it as the volts drop off would have a big drop in out put on the inverter,,,also thats one big mother of a fridge to use 3000 watts????
you need a small power station to run it,,and if it do use 3000 watts your inverter is far to small to do the job, read the out put plate on the inverter,,it says 3000 watts max? not all the time,,,,thats about 2000 watts realy running contently and the whire to feed the inverter will need to carry about 30 amps so your car battery is say 90 amp hours plus the volts drop off, you would be lucky to have the inverter running for 3-4 hours before your battery go's is very flat on you!

check the wires you either have a bad connection or a bad wire the power in verter has 4 wires 2 neg 2 pos if 1 of the 4 is loose or bad it will give lup code .

insufficient ground...check your connections and try again...do NOT continue to use the unit if the problem persists.
Good Luck