The simple answer is that this is an idiomatic form, and that la penso/i/... così is a standard way to say, more or less, “this is my opinion”.
To go deeper, in situation like this, la can be considered a fixed falso oggetto (“mock object”, say), that is, a pronoun that doesn't necessarily refer to a specific object mentioned previously. This is usual with the verb pensare, as here, and with other ones, such as farcela (to make it, to cope), prenderle (being beaten), bersela (to believe something untrue), avercela (con) (to be cross (with somebody)), prendersela (to be pissed off). Such verbs are known as procomplementari.
For more info, see the article “oggetto” from Enciclopedia dell'Italiano Treccani (where the above examples come from) and the paper “I verbi procomplementari tra grammatica e lessicografia”, both in Italian.
As for ci and ne, they both can be used with pensare, but the meaning is different. Pensarci means “to think about it”, and generally it's well clear about what:
Ci penso giorno e notte
meaning “I think about it night and day” (about something already mentioned). It can also mean “to attend to it”:
– Serve un cacciavite.
– Ci penso io.
that is, “We need a screwdriver” “I'll take care of it”.
Ne, in general, means “of it, about it” and you can of course use it with pensare. For instance:
– Che mi dici di Mario?
– Ne penso un gran bene.
(that is, “What about Mario?” “I think highly of him”, but more literally, “I think very well of him”).
To close, notice that it's quite frequent to use some of these pronouns in a redundant way, that is, using both the pronoun and the thing/person the pronoun refers to. For instance, in the last example, you could ask Che ne pensi di Mario?