At the beginning of combat on your turn, if you control an artifact, Toolcraft Exemplar gets +2/+1 until end of turn. If you control three or more artifacts, it also gains first strike until end of turn.
2016-09-20
If you control no artifacts as your combat phase begins, Toolcraft Exemplar's triggered ability doesn't trigger at all. If you control no artifacts as the triggered ability resolves, it has no effect.
2016-09-20
Whether or not you control three or more artifacts is checked only as the triggered ability resolves.