There's a lot to consider here. There are 3 of you + boss. The boss has shown little thought for you all, and is away for months, leaving you all with the business to run as normal - but no definitive contact or route to get directions from her if needed. Nobody is described as having powers delegated to let them act as a director in her absence, which could be a serious problem, or having clear knowledge about all aspects of the business, If just one of you three goes, the other 2 probably couldn't cope, and the scope for blame seems large if everything isn't perfect. For all we known, you and your 2 colleagues might see eye to eye and get on fine, or not work well together. And the pay is dire.
I'm not usually one to advise drastic action, but as it stands this isn't sustainable. Too much can fall foul in a 6 month period. But equally, you and your colleagues may not want to leave as some answers suggest.
What I'd be tempted to do instead depends on how likely it is that the 3 of you can get on and have a common viewpoint. Can the 3 of you talk safely and honestly with "cards on the table", without word getting back to the boss? If you are "on your own", you have fewer choices. But if the 3 of you can talk, then you have other options.
Specifically, pregnancy leave doesn't wait for people, so your boss is over a barrel. If the 3 of you want to continue working, you are in a position to decide what you need, to make that safe and possible. Some things would be nice (extra pay?). Others could be essential (legal indemnity/insurance; agreement in writing of what is allowed/not allowed and that you can run the company as directors in her absence; her statement that you are allowed to follow your own best judgment if a decision is needed and she can't be contacted - get a lawyer to draft this, DO NOT try and do it yourself, and company to pay).
The bottom line is, she needs you (3), and you (3) need to be safe and fairly treated and not at risk. Those 2 goals should be compatible - if you aren't safe then you can't reasonably be in legal control for that time, so you will almost certainly have to leave (all 3 of you) or take a risk that it'll all be okay "somehow" and cross fingers. Like anything in business if she wants it, she may have to pay for it.
If the 3 of you find you can reach a common position, then maybe it's time to write her a formal letter signed by all 3 of you setting out that position, and summing up what you feel and will need in order to take on the role she blithely proposes.