No, you can't. You cannot charge a 3 cell LiPo battery safely with any charger designed to charge a lead-acid battery, or a naked buck converter.
Really, you ought to get a smart charger will protect and extend the useful life of the cells significantly. But if you balance them once, you can probably get away with a more inexpensive LiPo charger. You do need to use a real, dedicated charger however or you will kill your cells very quickly (if they don't burn down your house before that) with the setup you've described. I imagine using a proper charger will pay for itself after just one set of batteries and the extra life you'll get, vs. using a buck converter.
Also, the behavior you describe in the comments:
CC at first until the cutoff voltage is reached then will change to CV which is just what is needed for LiPo's
... is, well, incorrect. That is not what they need. Not one manufacturer on Earth says that is how one charges their cells. It is not how any LiPo chargers charge cells. It is not how your laptop, cell phone, bluetooth speaker, battery powered widget of yore charges its cell(s). And it is not how you should charge them.
Because what you've described will overcharge the cell. Floating a lipo at 12.6V will overcharge the cells. As you know, this is something to avoid.
LiPos must be charge terminated if charging to 4.2V. If you don't do this, you are overcharging the cells. LiPo cells cannot be charged using a CC/CV charger, or certainly not a buck converter with CC/CV behavior. No LiPo charger works this way.
They are charged at a CC/CV with a C/10 charge termination. What that means is, at the end, they are charged at a constant voltage of 4.2V per cell, until the charging current falls below the C/10, after which the charge is terminated. However, they aren't just 'charged once', there is a voltage comparator that will automatically top-off the cells once they've self-discharged a meaningful amount (usually 95% SoC).
The reason here is that the LiPo cells never have a terminal voltage that is quite as high as the charging voltage. They are charged at 4.2V, but disconnect them, and the terminal voltage falls. On a new, fresh cell the difference can be very small, 50mV or even less, but as the cell is used and ages, the voltage drop will increase.
See, the 4.2V is a safe voltage to force current through the cells, and is not the float voltage. The float voltage is identical to the voltage the cells put out when not connected to any load or charger. But, this voltage is not particularly predictable or consistent over time and across cells, so it is generally not done. There are some high end, expensive energy-storage class devices that will periodically measure each cell's open circuit terminal voltage and make adjustments and float each cell at exactly the voltage it needs, and this works quite well. It's also expensive and is really only getting you a few extra % of stored energy, which usually only makes enough of a difference on very large scale industrial stuff.
So if you leave the cells connected without charge termination, you will be overcharging the cells, which is the quickest way to kill the cells, and is potentially dangerous. And if you have been charging LiPo cells under the belief that they can simply be CV'd at the end, please stop. That is incorrect. Charge must be terminated once the current falls to a certain level.
If they are large cells, you should really get a smart balancing charger. If you won't, I think that is a bad decision, but at the minimum, you need a proper LiPo charger, as it will have charge termination. And it will top off the cells periodically after they have self-discharged enough. This will significantly extend the life, be safe, and actually be correct and inline with every single battery manufacturer's usage requirements, as well as what we know is the correct way to use these cells. It isn't like there is a dearth of data about this stuff. There is a right way and a wrong way to charge these beasts, and what you're describing is the wrong way. Please use the right way.