How to cope with many languages at the same time

1) It is possible to lose proficiency in any language if not used. This may take many forms from total loss to reduced vocabulary or just decreased fluency.

2) & 5) There's no hard limit to the number of languages one can acquire. For example, many historians will learn 4 or 5 languages at quite high level to be able to read sources. For conversation, the limit is set more by opportunities and need to use the language. Learning and maintaining a language is a fairly labor-intensive and time-consuming but there doesn't seem to be a limit in terms of brain or memory limitations. Your goal of English, Finish and German is not all that extreme.

3) There are many strategies polyglots use. I'd recommend watching interviews with polyglots on YouTube. Like this one: https://www.youtube.com/watch?v=7CBlMYYf8Uk. Many of these were collected in the book 'Babel no More' (http://www.babelnomore.com). You can also read the Benny Lewis' blog entries on http://www.fluentin3months.com/blog. Using his methods is a plausible way to achieve proficiency but requires high commitment.

4) There's no set time. Once you achieve that level of proficiency, you don't need to maintain the languages so hard. Just make sure you read, listen, write and speak in each language once in a while. However, you will have to work very hard to get to that level. Still, you will have to discover how much time works for you.