The Joie i-Spin is not a 0-12y seat, it's 0-4 years. So either it's not the i-Spin or it's not up to age 12.
Do you know how tall she is?
Could you try her out in another kind of seat e.g. one that a friend has, or in a shop? I am thinking that something like Maxi Cosi Titan or Britax Evolvafix might be more comfortable for her (though, a pain to buy a new seat if you do have a 0-12 year one). The pads on the Joie seats are very "grippy" and I wonder if it's this she doesn't like.
Or at a push, the Cybex Pallas type seats which have the impact shield rather than the 5 point harness. Some children like these as there are no shoulder straps. There is no statistical difference in safety between these compared to a 5 point harness used forward facing.
If she's under 15kg or 100cm, then she's too small to go into a seatbelt (high back booster) legally. And TBH, there are iffy safety results for them near this minimum border. It is better to get them as close to 18kg/105cm as possible as a minimum really. And 3y4m is very young behaviourally to manage using just a seatbelt. In general most of the companies are now recommending 3.5y+ or 4y+ - in my experience fitting, where I worked we did fit from three and I rarely found a three year old that sat properly in them, certainly not the younger side of three. A handful of older 3s (nearly 4) but still not a lot of them.
Check you're not doing it too tight. It doesn't have to be so tight that it's hurting them. Also with the spin going both rear facing and forward facing, try the opposite way around? If she's FF currently, try rear facing saying it's a fun/silly/astronaut thing. If she's RF currently try forward facing.
Behaviour management wise, I would try to work with her to find out what she is struggling with and see if you can find a win-win solution you're both happy with. Do this at a no-pressure time so she knows that you're not going to force her into the seat, it tends to make them more likely to give you useful information. If you can do this e.g. combined with trying out different kinds of seats, brilliant as she might be able to tell you what she prefers about different kinds.