I've just returned from 7 nights in Valencia (including a wild evening in an Irish bar packed to the rafters with Germans watching Brazil being humiliated).
I want to clarify the position with the T1/2/3 ticket as it wasn't quite what I expected.
We walked to the Metro station in the airport and used the machines on the platform to buy tickets for the trip into the city centre. The machine has a language button - I scrolled through the options until I got English. I choose a single ticket, zone ABCD, and the screen informed me that I could put up to four persons on the one smartcard, so I chose two single tickets. The machine issued me with one cardboard smart card and charged me €8.80.
This worked fine at the security gate - I touched once and it opened for my mother and, after the gate closed, at the second touch it opened for me.
At Xàtiva Metro station I put the cardboard smartcard into the machine, hoping to charge it with two T3 tickets, but I was not offered the option.
It transpired that I had to buy two plastic smartcards from the ticket office. I also had the cards charged with a T3 ticket for each card. I can't remember whether there was a manned ticket office at the airport - if there was I could have got the plastic smartcards from there and used it for the single journey.
The 72 hours begins with your first use - this means you get 2 full days and 2 part days (mine went from 7pm on Monday to 7 p.m. on Thursday).
I recharged the smartcard before the original expired - this was fine. The recharge doesn't begin until you make the first use - I didn't travel after 7 p.m. on Thursday; my next journey was 10 a.m. on Friday, so my recharge didn't begin until then.
Each machine has an option to check your smartcard - it shows then date and time of expiry, as well as details of every journey that has been made.
One word of warning - if you want to charge a different ticket to the smartcard, you have to wait until the first one expires. I couldn't charge a T1 for the final day until my T3 had expired.
The smartcards are very easy to use - on the bus there is a pad behind the driver that responds to a light touch of the smartcard and a nice "beep" to show that it has been accepted. I made extensive use of the buses as my mother has mobility problems - they are excellent, but you need to plan ahead as all buses follow one circular route, that is, it does not have a bus following the route in the opposite direction. The EMT website is excellent for planning, showing all routes, bus stops, timetables etc.
Top marks to EMT.