Hi Francisco,
Google suggest to do the sitemap as your third option. Check this article:
https://support.google.com/webmasters/answer/2620865?hl=en
My way of doing it, is creating a sitemap for every language, uploaded all of them to the root of the site and create a sitemap stating the latter sitemaps.
This is an example:
<sitemapindex<span class="html-attribute"> xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"</sitemapindex<span>><sitemap></sitemap>https://www.domain.com/sitemap-en.xml<sitemap></sitemap>https://www.domain.com/sitemap-pt.xml<sitemap></sitemap>https://www.domain.com/sitemap-es.xml
Hope it helps.
GR.
