If the 3 gallon has a filter, that can be moved into a bigger tank - or the media 'persuaded' to fit inside a bigger tank's filter. However, there will only be enough bacteria to deal with the waste from 1 guppy so adding more fish means growing more bacteria. But if the bigger tank also had live plants - fast growing ones or floating plants rather than slow growers - they would help by removing a lot, if not all, of the ammonia made by the new fish.
The main problem with getting female guppies is fry. Each female will have fry every months. Not all of them, and maybe not any of them, will be eaten, so the tank would soon be grossly overstocked. I would stick with more males.
The only cories suitable for 10 gallons are the three dwarf species. Cories must be kept in groups of at least 6, and 10 gallons is too small for a group of the larger species.
Cories are also soft water fish while guppies are hard water fish. Other members have asked about the water hardness, but we've not had an answer yet.
@Teddy C can you find out how hard your water is please. Look on your water provider's website to see if they give it - you need a number and the unit of measurement. If they don't give it, take a sample of tap water to a fish store and ask them to test it got GH. Make sure they give you a number. If it turns out you have soft water, it would be better to get soft water fish and while it's not ideal for Marco, just let him live out the rest of his life in a bigger tank with soft water tank mates.
On the subject of testing water, we haven't established why the other two guppies died. It is not advisable to get more fish whatever the tank size until we know why they died. Can I ask a few questions:
How long has the tank been set up?
Did you cycle it before getting the guppies - in other words did you add ammonia until the good bacteria had grown?
What are the ammonia and nitrite levels in the water?
Were the guppies ill before they died? Did they show any signs such as white spots, white patches, ragged fins, swollen bodies, whatever?