I used to play all triads of the left hand with fingers 2-4-5 instead of 1-3-5
Is this acceptable or do I have to practice 1-3-5?
You should practice playing with 1-3-5, not because it is the "proper" way, but because when playing a piece you will need to change from chord to chord, chord to melody or bass line, or a more complex arrangement, and generally it's not good to keep the same fingering for all chords.
In general, changing between chords should minimize the number of note changes (that kind of reasoning is called "voice leading") and hand and finger movement. So you would play one chord with 1-3-5 and the next one 1-2-4, or vice-versa, depending on circumstances.
For example say you're playing Cmaj in root position (C-E-G) and next you want to play Fmaj. Rather than jump your hand to F-A-C, you could keep the common C and play the F chord as C-F-A (that kind of reasoning is called "voice leading"). The more natural fingering would then be (for the right hand):
But if that was with your left hand:
(Thanks to Bruce Fields for correcting a typo in the examples).
These are just examples, there is no single right way to finger a piece, you should find the fingering that best suits you, but making an effort to minimize hand and finger movements.
As usual, about fingering, it all depends of the context. Sometimes you could even get to play them 5/4/3 or 1/2/3 .
However, on a general basis, without other reasons, you should indeed practice 1-3-5. As mentioned by joseem, this is the proper way to play them, and you'll eventually come to that point by mastering technique. Purpose of fingering is to get you there a bit faster, in fact.