Having recently installed /e/ on both an A5 - quite easily - and an A3 but with more problems that I related here more due to problems with the Heimdall and the recent TWRP that I flashed, I would say, as @Rik does, that the A3 could suffer a little with intensive browser use as it has half a gig less of RAM, but that said in ordinary usage I doubt that it makes a great deal of difference - certainly the person I flashed it for has no complaints and is a definite improvement on the MotoG she had before, on top of that her smaller feminine hands appreciate the smaller and thinner size of the A3.
Both devices are running the latest Pie version of Android and the only difficulty that I had was with the synchronisation of her e-account as there seems to be a problem with importing vcf files for the Contacts app. This has been evoked elsewhere and seems to be a bug affecting the ecloud service and not the devices themselves. Unlike the older Oreo version the new Contacts app doesn’t seem to be able to create contact groups, and as you seem to be a French user, I would be careful of accented characters in your contact list. I suspect that the export function you might use to save your contacts doesn’t handle non-Unicode characters and when I repeatedly tried to import the vcf file on the ecloud site it failed each time until I removed all trace of French accents.
Obviously this might be an important function for non-QWERTY users in other languages as well, so I would advise trying to convert a vcf file exported via the Contacts app using an online conversion tool to a csv file for example. If you get an error message or unreadable output then my suspicions will be proved right.
Having experimented recently with the ecloud server I discovered that its export function seems to work correctly, so perhaps just relying on a synchronisation update might work after installing /e/ on a new device.
By the way, the A5 I bought from Backmarket’s French website. Less than 150€.