hello It depends on the context. You can use "a través de" for instance in this sentence " Puedo ver a través de la ventana" (I can see through the window), but you cannot say "Puedo ver por medio de la ventana". You can use both phrases in the case you mean "by means of". "a lo largo de" you can use for making refer of time, relationships, distance. I hope this helps, if you need further explanation or examples, just ask.

i am not a native speaker but i think por medio de is 'by means of', a traves de is 'through' and a lo largo de is 'during or throughout'....there are all similar but they are used the same way the english counterparts function in english

Egwu's answer pretty much sums it up well. Por medio de: Through this/By means of (Very formal I will add, usually found in legal or business language, and normally only written) A lo largo de: "Over the (years, months, weeks etc.)" A través de: This is just "through" or "via" I would say they are actually pretty different to Spanish ears. The first one as I say is the most formal one, the other ones are less so. Hope this helps.