Ability Tests in OD&D

As far as I can see so far (remember - I am reading the LBB from front to back completely for the first time in my life) OD&D does not contain any actual rules for ability tests and the like. This is something I am going to elaborate slightly for 'The Fantasy Game'.
I am going to use a rule that I view a natural extension of the character creation rule. Since abilities are determined by rolling 3d6 (or 4d6, take the three best under my modified rules) I will use 3d6 to roll ability tests. The natural distribution of 3d6 makes high ability scores a lot more valuable than average ones. If you are interested in a little statistics, you can find some details about the differences between d20 and 3d6 here.

So here is the basic outline for ability tests in 'The Fantasy Game':

• Roll 3d6. If the resulting score is equal to or less than the ability score in question the test succeeds.
• A roll of 3 always succeeds.
• A roll of 18 always fails.
• I'm pondering to introduce a rule for critical success or failure if all three dice show the same result (e.g. it's a critical success if all three dice show the same result and the roll is a success, it's a critical failure if all three dice show the same result and the roll is a failure).
• Modifiers are either +1 or -1 for minor advantages or disadvantages and +4 or -4 for major advantages or disadvantages.

Ability tests in 'The Fantasy Game' are used for raw ability tests (e.g. rolling a strength test in order to break down a door), raw talent tests (e.g. rolling a dexterity test in order to climb a tree) and skill tests (e.g. trying to sneak past a guard). Note that 'The Fantasy Game' will not include any kind of skill system... the viability of a skill test is determined by the referee who needs to judge its merits based on the race, background, profession and experience of the character attempting the feat. Totally free form. There might be optional side notes for referees who want more detail but there won't be much more than a few lines of additional advice.

Oh, and I forgot to talk about why I did not consider using a d20 for the ability tests: I'm still bound to make 'The Fantasy Game' a "d6 only" game as it seems to me very appropriate for one of the early games to no put up hindrances in the form of weird dice. This also fits nicely to the Chainmail combat system. Thus using 3d6 for ability tests seemed to be very natural.