I am in my late 20s and my bto is due for completion in early 2022. From now till then, I have roughly 3-4 years.
OK. So you and your significant other will need a chunk of money for the down payment. Ideally you'll want to have some cash so that you could choose a bank loan if you wish, but CPF Ordinary Account funds can also be used. SSBs work great for the cash portion of your down payment.
1. Should I focus on saving up my monthly spare cash (in bank account) into a lump sum and reduce my housing loan amount when bto is ready? This projected saving over the period works up to around 70-80k.
At current and even somewhat higher interest rates, no. If you expect to be able to qualify for the HDB loan rate of 2.6%, that's really the "worst case." Last I checked, market interest rates would still have to rise more than 2 percentage points before the HDB concessionary loan rate would rise. And, even if that happened, Ordinary Account interest rates would rise, too. Mortgages are still inexpensive in Singapore, and so I think it's still perfectly reasonable to take, say, a 75% mortgage (25% down payment).
The interesting question 3 to 4 years from now is whether you should take the HDB concessionary loan rate (2.6%), if you qualify. That rate is looking somewhat more attractive these days. But I think I'd still put 20% down on a HDB loan, if you can reasonably afford it. And you can decide that question 3+ years from now.
This all may seem counterintuitive, but it is correct.
Cheap debt is great, while it's cheap. If you can borrow money at 1.8% fixed for 3 years and take that money over to ICBC Singapore, deposit it in a fixed account, and get 1.85% -- and some people really can do this -- then of course it makes sense to accept the cheap money and not to accelerate loan repayment. Effectively the banks are paying you (in this example) to take their loan, and that's ridiculous and crazy, but if that's the offer, take it! That's just an extreme example, but certainly if you can put your money to work and reliably earn 3% or more (let's suppose), even a 2.6% mortgage is cheap. So I'm not opposed to cheap debt, but I like to be well defended if that debt turns into something that's no longer cheap. Your savings and investments, per normal long-term practices, should defend you well against that risk.
2. Invest my current monthly spare cash (around ~2k) into IWDA, ES3 and SSB? In the suggested ratio.
I'm not sure which suggested ratio, but, regardless, it should be a ratio that you like for your goals. Yes, while mortgages are cheap, that's a very reasonable bet to make.
I am incline to do the first as in general I don't like to be on high debt and home loan interest rate seems to be going up.
Yes, I know we don't like "high debt," but we should like cheap debt while it's cheap. So for now just focus on making sure you have a decent down payment -- maybe you do already -- check the mortgage rates every once in a while, and see where we are 3+ years from now, especially with respect to the HDB concessionary loan rate if you qualify. If the banks 3+ years from now are lending at 2.5% fixed for 2 or 3 years and HDB is lending at 2.6%, let's suppose, I'd probably take HDB's offer. (This is an example.)
If you've got enough for a 20% or 25% down payment now, or you will based on your normal savings pattern, that's enough I'd say. Then, 3+ years from now, if banks are loaning at 3.5% and you don't qualify for HDB's 2.6% rate, you might decide to sell some ES3 (for example) to increase your down payment from 25% to, say, 40%. (Why focus on ES3 in that event? Because your new home is also in Singapore, exposed to similar valuation risks. So to maintain a similar global diversification level, you'd probably sell some ES3 if you wanted/needed to raise more funds to increase your equity stake in your new home, if and only if mortgages are genuinely expensive.) That could be a reasonable thing to do. But I don't think it's worth changing your portfolio allocations now for that possibility, not yet anyway.