At 3 I would not go for an impact shield seat. She'll have outgrown it in height soon making it dangerous, as the child is only pinned by their pelvis basically - yet there are no actual height restrictions unlike with a harness - they are only tested with standard sized dummies so you don't get this effect. I do think they have a place but it's not this one.
I would not use a houdini strap or similar, because this interferes with how the harness works and could potentially be lethal in an accident. Soft items which exert no force on the straps, like the belt collector or the 5 point plus are safer.
Personally I think if you have an escape artist then RF to 25kg becomes a lesser concern, because visibility is important - I know tea and I disagree on this :)
Do you know how your DD is escaping? 5pp have a video on their site showing how they do it, by sucking in their stomach and getting arms through! I do think the 5pp is probably the best product for this. If it's more that she's undoing the harness, you can try sticking the hard part of velcro to the seat button. That makes it painful for small children to press the button but an adult can still do it.
I don't think buying a new seat is likely to help, for this reason. The Cosatto ones only harness to 18kg which will be the same as your current seat, so there's no benefit in getting that, you'd be better off with the 5pp as an aftermarket accessory. The impact shield I've already explained my issues with. You could look at a seat which harnesses to 25kg if she's getting close to outgrowing her current harness and you don't yet trust her with a booster seat, particularly if you find a harness-based solution that works, but there's nothing to gain from this yet if you try the 5pp on her current seat first.
Otherwise - you can look at it behaviourally. Distraction can help if she is bored, especially combined with something which makes it just slightly more difficult to escape. Some people have luck with bringing out the big guns and really scaring the child so that they don't want to do it again. When DS1 did it he was old enough to reason with so we explained about the safety aspect and told him if he waited until we said so, he'd be allowed to undo the belt by himself, but if he didn't wait he wouldn't be allowed any more. That worked for us (and might be useful if you do decide to move to a booster seat soon.)