OK at just turned 3 and 14kg they are definitely too little to go into a seatbelt booster, so you'd need a harnessed seat. Most likely a 123 type seat will be best, as these have the 5 point harness up to 18kg or 25kg then turn into a booster seat for later.
Your isofix base won't take one of these types of seat, they only work for baby seats and sometimes the toddler one that goes with the set.
I really like the Britax Advansafix i-size, but you'll probably want to check LO's height as it has a height limit of 102cm for the harness, if they are already over about 97cm then this won't last long enough. If this is the case then you could look at the older Advansafix IV which has an 18kg harness limit but no stated height limit on the harness and fits until about 109cm.
You need to keep LO in a 5 point harness until they are at least 15kg and 100cm, though preferably closer to 18kg and 105cm, and also until they're mature/sensible enough to be trusted with the seatbelt which probably won't be until about 4 or sometimes older.
If you want to keep the 5 point harness for longer, or your LO is already close to 105cm tall, then the Joie Bold is worth looking at as PP said as this has a harness up to 25kg and equivalent height, which is about 125cm or a bit taller.
All of these need to use both isofix and top tether to fit. You MUST use the top tether so you need to check if you have the right anchor points in your car. If it's less than 10 years old, then it will have them, but if it's older you'll need to look in the boot (and maybe in the manual) and check.
If you don't have top tether the Recaro Tian Elite is the best rated one, but if that's too expensive look at the older, seatbelt fit Britax Evolva.