It depends on how in-depth you want the trip to be. 12-15 days in Portugal will allow you to see a great deal, but you'll still be leaving out some very nice places. In the other two countries you will be much more limited. I tackled Spain in 2016; I spent 89 days in the country (albeit without a car, which makes me slower) and I did not include Andalucía. I'm going back this year to see Andalucía and a few other spots I missed, like Valencia and Palma de Mallorca. I haven't figured it all out yet, but it looks like it will be another 45 days or so. And I have absolutely no interest in several things many travelers spend considerable time on: beaches, wine and palaces.
Since Italy is the farthest afield (though traveling on the ground between Spain and Portugal is a significant challenge, as you will find out), I'd suggest focusing first on Spain. Get a comprehensive guidebook and did into the chapters on areas not already on your mental target list. Are you OK with skipping the Basque Country? Galicia? Everything in Catalunya except Barcelona? Two weeks is not really enough time for Barcelona-Madrid (Toledo and Segovia day-trips), Cordoba, Seville and Granada. Based on how you handled Germany, I don't think you'd be happy with 14 days in that set of cities I listed, so you could either pare the list back, focus on a small section of the country, or drop Italy from this trip and use that time mostly in Spain. That would still not allow you to cover the full extent of the country, but the omissions would be less painful.
As for Italy, I don't even know what amount of time I'd suggest there. Maybe six months (not legal without a visa) would be a decent start. Again, it's all in what you want to see. Two weeks would be OK for Venice-Florence-Rome, and you could do a side-trip or two in Tuscany without rushing. But you're missing the Dolomites, the lakes, Bologna and all its side-trips, Verona/Vicenza/Padua, Turin, Milan, Naples and environs, the Amalfi Coast, Puglia, Sicily (worth 3 weeks by itself).
Edited to add: Even Portugal, small though it is, could probably fill all your time if you included Madeira, which is drop-dead gorgeous but would require flying in and out.