Southern Spain: Granada, Cordoba or Seville?

Are you saying that you are staying 6 days in Marbella and looking to do day trips to one of those 3?

Of the three towns you mentioned, I would choose Granada and Sevilla. Granada has the Alhambra (a Moorish castle with beautiful interior design) which is amazing. The rose and other floral gardens also were beautiful. The town has charming narrow lanes which are fun to wander. Sevilla is what I imagined Spain to be like. It's subtropical with lush greenery. There are iron worked balconies in the historic section, flamenco dancing shows and more. Take a walking tour if you can.

With the right guide, or if you have the eyes to see, you'll notice more vestiges of al-Andalus in Granada as well as more of today's thriving Andalusi cultural revival than in Sevilla. But you can't go wrong in either.

https://www.jstor.org/stable/23076032

Just do your prep beforehand!

No way would I miss the Mosque in Cordoba. All three choices are incredible. You could base the two on the ease of transportation from where you are staying.

Cordoba is delightful, and the mosque-Cathedral it here is amazing. But to answer the question that was asked, if I could only do two of the three, it’s what I’d miss.

Granada and Seville.

You won’t be disappointed whichever you choose. Each is worth visiting and are all within a 2.5 hours drive from Marbella.

Tickets to the Alhambra often sell out really early. It's unlikely you'll be able to wait till you're in Spain to get those tickets. When is your trip?

You also don't want to walk up to the Alcazar in Seville or the Cathedral without a pre-purchased ticket. Rick's guidebook is extremely useful in its coverage of issues like these.

Marbella has no train station, so driving will be the fastest way to get to the other cities. Check into the parking situation ahead of time to avoid wasting time and money upon arrival.

ViaMichelin.com will provide route information. These are its driving-time estimates for the fastest of the suggested routes:

Granada: 1 hr. 59 min.
Cordoba: 2 hr. 20 min.
Seville: 2 hr. 47 min.

Those times don't include stops along the way, navigational errors, traffic tie-ups, looking for parking, or getting from your parking place to what you want to see. It will be a lot of driving, and every one of those cities is worthy of being more than a day trip. Seville, in particular, is a large city with a lot of sights. I'd usually recommend at least four nights there.

I assume you already have a day trip to Malaga planned. It ranks behind the other three cities, but it is a worthy destination, and the driving time is estimated at less than an hour. Driving time to Ronda, another interesting--though lesser--destination, is estimated at 64 minutes. As someone who's uncomfortable not having enough time to see the key sights at an enjoyable pace, I'd be tempted to stick to places like Malaga and Ronda this time and plan another trip to Spain when I'd have more time and could stay in the cities I most wanted to see.

With those substantial driving times, those should be overnight stops.