Hey there,
There is tons of reasons why emails would go to spam, matter of fact, there's so many spam filters out there that we can never be sure that 100% of our emails will reach our users' inbox.
A study by ReturnPath stated that 21% of emails sent by marketing campaigns go to the spam folder ([login to view URL])
The biggest e-mail marketing companies out there such as MailChimp face the same problem.
It's not about how many or how fast you're sending emails, it's purely about the email itself and, most importantly, the recipient.
Spam Filters will drop your email if it's coming from an unauthorized server, if your IP Address is blacklisted, if your audience isn't yours or if you're targeting them in a bad way and if you're using a complicated or messy template.
It's also important to note that sending an email 3 times a year, like you stated, will most likely get your own audience to forget about you and move your email to SPAM which will trigger Spam Filters to attack the rest of the emails you've sent. ([login to view URL])
There is no magic way of doing this, we just have to "comply" the best way possible, get more personal with the audience, update the template accordingly, maybe even get the audience to add us to their address book for example. We keep updating this "setup" after every campaign to make sure the next one will get even better results.
Mario A.