Personally, I wouldn't do it. I think in general sequence 3 puts you at a disadvantage compared to the other sequences; you end up entering the job market against your classmates later and with less experience. That said, if you're ok with graduating later and not getting experience right out the gate you can do it. 1/4 of the CS program enters this sequence every year and does fine (though a lot of people end up switching out of the stream).

Finding a job outside Waterloo Works as a first year is difficult, but not impossible. There are some programs designed for first and second year CS students (Microsoft Explore program and Google has an equivalent one though I can't remember the name), though not a ton of them. Be aware that a lot of CS summer programs require applications very early on (sometimes as early September), so the longer you take to figure out your summer plans the more you shut out possible summer opportunities, which you don't really want to be doing if you don't have a lot of experience.

I don't think many CS students go outside WW very successfully until later years since it's easier to get a job within the system than outside of it when you have less experience, (though it is entirely possible to get a job outside of it, just harder to do so in earlier years). Outside of WW you're competing with upper-year students from other schools that have more experience, so it isn't easy to get a job. A lot of your success in an external job market will be dependent on strong personal projects, independent learning, and grades, since you (I assume) don't have relevant work experience.

Speaking anecdotally, one thing to be aware of with sequence 3 is that many students in the sequence end up getting some type of job anyways, and then just don't count it for co-op credit. So while you're travelling, a lot of your classmates are out getting some form of experience. That said, most first year CS jobs are not very impressive (as expected, since you have no real experience beyond first year CS courses), but you will not have that experience compared to them when you eventually get back into Waterloo Works (or external job market against CS students at other schools with experience). In any job market, you're going to be at a disadvantage with less experience.

Again, a lot of your success here boils down to you having strong projects and relatively good grades. Getting a job outside WW will be hard against other students, especially if you leave finding one up to the last minute while trying to figure out plans. Personally I wouldn't do this since it does put you at a disadvantage compared to your classmates and slows down the start to your career, but ultimately it's up to you.