Let's use Leona as an example, even though she realistically won't go top lane.
At level 3, Leona's base armor (which is the highest in the game starting at lvl 1) and armor growth would give her 52 armor. Add the +12 armor from stat shards to that and Leona can have as high as 64 armor at level 3 (if she doesn't start a cloth armor.)
64 armor gives about 39% physical damage reduction. Assuming Renekton hits you with an empowered E, that armor is reduced to 48 and instead reduces 33% of physical damage.
With 75 damage (base damage + 3 levels of AD/level) and 12 more AD from stat shards, Renekton's full combo does:
Q - 65 (+70) = 135 reduced to [90]
W - 10 (+112.5) = 123 (rounded up from 122.5) reduced to [82]
W AA reset - 87 reduced to [58]
E (Slice) - 40 (+78) reduced to [79]
Empowered E (Dice) - 117 (+70) reduced to **[125]
**
##For a grand total of [434] damage post-mitigation.
At level 3 and with a Doran's Shield, Leona's max health would be 705. If you ate a full Renekton combo it would only deal 61.5% of her max health, which really isn't that shocking considering that Renekton is a highly aggressive laner.