The resulting artifact creature will be able to attack on your turn if it's been under your control continuously since the turn began. That is, it doesn't matter how long it's been a creature, just how long it's been on the battlefield.

If the artifact token was already a creature, its base power and toughness will become 3/1. This overwrites any previous effects that set the creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after The Mechanist, Aerial Artisan's ability resolves will overwrite this effect.

The Mechanist, Aerial Artisan's activated ability doesn't remove any abilities the target artifact token has. The artifact token also retains any types, subtypes, or supertypes it has.

Effects that modify a creature's power and/or toughness, such as the ones created by Titanic Growth or a +/1+1 counter, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness and effects that switch power and toughness.