How Can We Move Navigation Block to the left side i.e, below the dashboard block in moodle version 3.2.4?
Moving Block to the left side
Not in the standard Boost theme you can't.
If you want the previous '3 column' look there are plenty of other themes available. The core 'More' theme for a start.
I was pleasantly surprised a month or so ago to discover that Mary Evans has a preset for Boost that gives me a three column look very similar to what I am used to using. So far, it has worked very well for me. It is named preset_morecandy.scss and you can get it from her github repsoitory at https://github.com/lazydaisy/my-moodle-presets/tree/master/preset-morecandy
Might be worth a look.