3 STR : 1 Dex means 25% of your stat points should go to dex. Every 4 levels you get, you add 3 str and 1 dex.
At lvl 280: 314 total points (35 bonus stat points).
To get 3 STR : 1 dex, you divide 314 by 4 to get a number (the number = 25% of the total).
Then add the value to dex (25%).
The remaining points you add to STR (75%).
Now you have 3 STR : 1 Dex, or 75%str:25%dex.

For you, lvl 200, with total 234 points:

The benefit of high str, low dex is you may supplement crit rate with equips. Getting physical damage + critical damage on gear is more difficult (two stats). Getting crit rate is easier (green gems on weapon). The important stat from dex is crit rate.
Evasion/dodge is defensive, accuracy is not an issue for PvE.
High str grants you more damage, and more crit damage.
Yes, high crit rate (crit rate doesn’t have to be through dex) favours skills with high damage and high modifiers. Skills such as scout split arrow (200% modifier), cannoneer skills, musketeer skills, etc. You’d increase damage more when you crit on a skill which hits 10k (crit dmg 15k) than you’d when you add +500 STR damage (10k500 damage).
Quote:
If your skill damage is higher than 3500, critting is strong.
Relevant: