The perfect plan is that in your first year you should do at least one ‘Spring Week’ with an employer and aim for a summer internship with them at the end of your second year. Next, at the start of your second year, you should apply for other summer internships to hopefully allow you an assortment to choose from.
Complete your choice summer internship with your whole heart as you could just be offered a full-time graduate role with that employer if you do a well enough job. Armed with a job offer for when you graduate, you can still apply at the start of the third year if you were unsuccessful during your internship or would like to have a spread of a few selective full-time graduate roles.
Consequently, if you’ve worked incredibly hard, in your third year, you are sitting pretty with a fair number of elite job offers and when applying keep a log of all the application status & action. Save all your cover letters & applications materials as they can be re-used and re-purposed for future job propositions.